diff --git a/src/actions/assignment.actions.ts b/src/actions/assignment.actions.ts
index f9181a6..4c50ba4 100644
--- a/src/actions/assignment.actions.ts
+++ b/src/actions/assignment.actions.ts
@@ -6,6 +6,7 @@ import { localizeAssignmentFieldErrors } from "@/actions/assignment.messages"
 import { getI18n } from "@/i18n/server"
 import {
   buildCreateAssignmentSchema,
+  buildReturnAssignmentSchema,
   buildUpdateAssignmentSchema,
   type CreateAssignmentFormType,
   type ReturnAssignmentFormType,
@@ -120,14 +121,32 @@ export async function updateAssignment(formData: UpdateAssignmentFormType) {
   }
 }
 
-export async function returnAssignment(formData: ReturnAssignmentFormType) {
+type ReturnAssignmentActionResult =
+  | { success: true; message: string }
+  | { success: false; errors?: Record<string, string[]>; message?: string }
+
+export async function returnAssignment(
+  formData: ReturnAssignmentFormType,
+): Promise<ReturnAssignmentActionResult> {
   const { dictionary } = await getI18n()
   const copy = dictionary.inventory.assignments
   const userId = await getAuthenticatedUserId()
 
+  const validatedFields = buildReturnAssignmentSchema(copy.schema).safeParse(
+    formData,
+  )
+
+  if (!validatedFields.success) {
+    return {
+      success: false,
+      errors: flattenError(validatedFields.error).fieldErrors,
+    }
+  }
+
   const result = await returnAssignmentUseCase({
-    id: formData.id,
+    id: validatedFields.data.id,
     actorId: userId,
+    returns: validatedFields.data.returns,
   })
 
   if (!result.success) {
diff --git a/src/app/(dashboard)/assignments/_components/return.button.tsx b/src/app/(dashboard)/assignments/_components/return.button.tsx
index 530d471..b5f6bb3 100644
--- a/src/app/(dashboard)/assignments/_components/return.button.tsx
+++ b/src/app/(dashboard)/assignments/_components/return.button.tsx
@@ -2,53 +2,219 @@
 
 import { ArrowLeft } from "lucide-react"
 import { useRouter } from "next/navigation"
-import { useTransition } from "react"
+import { useState, useTransition } from "react"
 import { toast } from "sonner"
 import { returnAssignment } from "@/actions/assignment.actions"
 import { Button } from "@/components/ui/button"
+import {
+  Dialog,
+  DialogContent,
+  DialogDescription,
+  DialogFooter,
+  DialogHeader,
+  DialogTitle,
+  DialogTrigger,
+} from "@/components/ui/dialog"
+import { Input } from "@/components/ui/input"
 import type { ReturnAssignmentFormType } from "@/schemas/assignment.schema"
 
+type PartialReturnCopy = {
+  title: string
+  quantity: string
+  quantityPlaceholder: string
+  notes: string
+  notesPlaceholder: string
+  submit: string
+  cancel: string
+  maxQuantity: string
+  errorConcurrent: string
+  errorGeneric: string
+}
+
+const defaultPartialReturnCopy: PartialReturnCopy = {
+  title: "Devolver artículo",
+  quantity: "Cantidad",
+  quantityPlaceholder: "1",
+  notes: "Notas",
+  notesPlaceholder: "Notas opcionales",
+  submit: "Devolver",
+  cancel: "Cancelar",
+  maxQuantity: "Máximo: {max}",
+  errorConcurrent:
+    "La devolución fue modificada por otro usuario. Recarga e inténtalo de nuevo.",
+  errorGeneric: "Error al procesar la devolución",
+}
+
 export default function ReturnButton({
   assignmentId,
   ariaLabel,
+  assignmentLineId,
+  remainingQuantity,
+  copy = defaultPartialReturnCopy,
 }: {
   assignmentId: string
   ariaLabel: string
+  assignmentLineId?: string
+  remainingQuantity?: number
+  copy?: PartialReturnCopy
 }) {
   const router = useRouter()
   const [isPending, startTransition] = useTransition()
+  const [open, setOpen] = useState(false)
+  const [quantity, setQuantity] = useState(1)
+  const [notes, setNotes] = useState("")
+  const [errorKey, setErrorKey] = useState<
+    "errorConcurrent" | "errorGeneric" | null
+  >(null)
+
+  const isQuantityMode =
+    assignmentLineId !== undefined && remainingQuantity !== undefined
+  const isOverMax = isQuantityMode && quantity > remainingQuantity
+  const canSubmit = isQuantityMode
+    ? quantity >= 1 && quantity <= remainingQuantity && !isPending
+    : !isPending
+
+  const handleOpenChange = (nextOpen: boolean) => {
+    setOpen(nextOpen)
+    if (!nextOpen) {
+      setQuantity(1)
+      setNotes("")
+      setErrorKey(null)
+    }
+  }
+
+  const handleSubmit = (event: React.FormEvent<HTMLFormElement>) => {
+    event.preventDefault()
+    if (!canSubmit) return
+
+    setErrorKey(null)
+
+    const formData: ReturnAssignmentFormType = isQuantityMode
+      ? {
+          id: assignmentId,
+          returns: [
+            {
+              assignmentLineId,
+              quantity,
+              notes: notes.trim() || undefined,
+            },
+          ],
+        }
+      : { id: assignmentId }
 
-  const handleReturn = (formData: ReturnAssignmentFormType) => {
     startTransition(async () => {
       const response = await returnAssignment(formData)
 
-      if (!response.success && response.errors?.id) {
-        toast.error(response.errors.id[0])
+      if (response.success) {
+        setOpen(false)
+        setQuantity(1)
+        setNotes("")
+        toast.success(response.message)
+        router.refresh()
         return
       }
 
-      if (response.success) {
-        toast.success(response.message)
-        router.refresh()
+      if (response.errors?.error?.includes("errorConcurrent")) {
+        setErrorKey("errorConcurrent")
       } else {
-        toast.error(response.message ?? "Unknown error")
+        setErrorKey("errorGeneric")
       }
     })
   }
 
   return (
-    <form action={() => handleReturn({ id: assignmentId })} className="w-full">
-      <input type="hidden" name="id" value={assignmentId} />
-      <Button
-        type="submit"
-        className="btn btn-error"
-        size="icon"
-        variant="outline"
-        aria-label={ariaLabel}
-        disabled={isPending}
-      >
-        <ArrowLeft />
-      </Button>
-    </form>
+    <Dialog open={open} onOpenChange={handleOpenChange}>
+      <DialogTrigger asChild>
+        <Button
+          type="button"
+          className="btn btn-error"
+          size="icon"
+          variant="outline"
+          aria-label={ariaLabel}
+          disabled={isPending}
+        >
+          <ArrowLeft />
+        </Button>
+      </DialogTrigger>
+      <DialogContent>
+        <form onSubmit={handleSubmit}>
+          <DialogHeader>
+            <DialogTitle>{copy.title}</DialogTitle>
+            <DialogDescription className="sr-only">
+              {copy.title}
+            </DialogDescription>
+          </DialogHeader>
+
+          <div className="grid gap-4 py-4">
+            {isQuantityMode && (
+              <>
+                <div className="grid gap-2">
+                  <label
+                    htmlFor={`quantity-${assignmentId}`}
+                    className="text-sm font-medium"
+                  >
+                    {copy.quantity}
+                  </label>
+                  <Input
+                    id={`quantity-${assignmentId}`}
+                    type="number"
+                    min={1}
+                    max={remainingQuantity}
+                    value={quantity}
+                    onChange={(event) =>
+                      setQuantity(Number(event.target.value))
+                    }
+                    placeholder={copy.quantityPlaceholder}
+                    aria-invalid={isOverMax || undefined}
+                  />
+                  {isOverMax && (
+                    <p className="text-destructive text-sm">
+                      {copy.maxQuantity.replace(
+                        "{max}",
+                        String(remainingQuantity),
+                      )}
+                    </p>
+                  )}
+                </div>
+                <div className="grid gap-2">
+                  <label
+                    htmlFor={`notes-${assignmentId}`}
+                    className="text-sm font-medium"
+                  >
+                    {copy.notes}
+                  </label>
+                  <textarea
+                    id={`notes-${assignmentId}`}
+                    value={notes}
+                    onChange={(event) => setNotes(event.target.value)}
+                    placeholder={copy.notesPlaceholder}
+                    className="min-h-[80px] w-full rounded-md border border-input bg-transparent px-3 py-2 text-base shadow-xs outline-none transition-[color,box-shadow] placeholder:text-muted-foreground focus-visible:border-ring focus-visible:ring-[3px] focus-visible:ring-ring/50 disabled:pointer-events-none disabled:cursor-not-allowed disabled:opacity-50 md:text-sm dark:bg-input/30"
+                  />
+                </div>
+              </>
+            )}
+            {errorKey && (
+              <p className="text-destructive text-sm" role="alert">
+                {copy[errorKey]}
+              </p>
+            )}
+          </div>
+
+          <DialogFooter>
+            <Button
+              type="button"
+              variant="outline"
+              onClick={() => setOpen(false)}
+              disabled={isPending}
+            >
+              {copy.cancel}
+            </Button>
+            <Button type="submit" disabled={!canSubmit}>
+              {copy.submit}
+            </Button>
+          </DialogFooter>
+        </form>
+      </DialogContent>
+    </Dialog>
   )
 }
diff --git a/tests/unit/actions/assignment.actions.test.ts b/tests/unit/actions/assignment.actions.test.ts
new file mode 100644
index 0000000..c0b707d
--- /dev/null
+++ b/tests/unit/actions/assignment.actions.test.ts
@@ -0,0 +1,104 @@
+import { beforeEach, describe, expect, it, vi } from "vitest"
+
+import { en } from "@/i18n/dictionaries/en"
+
+const mocks = vi.hoisted(() => ({
+  revalidatePath: vi.fn(),
+  getI18n: vi.fn(),
+  getAuthenticatedUserId: vi.fn(),
+  returnAssignmentUseCase: vi.fn(),
+}))
+
+vi.mock("next/cache", () => ({
+  revalidatePath: mocks.revalidatePath,
+}))
+
+vi.mock("@/i18n/server", () => ({
+  getI18n: mocks.getI18n,
+}))
+
+vi.mock("@/services/auth.service", () => ({
+  getAuthenticatedUserId: mocks.getAuthenticatedUserId,
+}))
+
+vi.mock("@/use-cases/assignment.use-cases", () => ({
+  returnAssignmentUseCase: mocks.returnAssignmentUseCase,
+}))
+
+import { returnAssignment } from "@/actions/assignment.actions"
+
+describe("returnAssignment action", () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+    mocks.getI18n.mockResolvedValue({ dictionary: en, locale: "en" })
+    mocks.getAuthenticatedUserId.mockResolvedValue("user-1")
+  })
+
+  it("returns validation errors for a missing assignment id", async () => {
+    const result = await returnAssignment({ id: "" })
+
+    expect(result).toEqual({
+      success: false,
+      errors: {
+        id: [en.inventory.assignments.schema.idRequired],
+      },
+    })
+    expect(mocks.returnAssignmentUseCase).not.toHaveBeenCalled()
+  })
+
+  it("forwards returns to the use case and revalidates on success", async () => {
+    mocks.returnAssignmentUseCase.mockResolvedValue({ success: true })
+
+    const result = await returnAssignment({
+      id: "assignment-1",
+      returns: [
+        { assignmentLineId: "line-1", quantity: 2, notes: "Slightly damaged" },
+      ],
+    })
+
+    expect(result).toEqual({
+      success: true as const,
+      message: en.inventory.assignments.actions.returnSuccess,
+    })
+    expect(mocks.returnAssignmentUseCase).toHaveBeenCalledWith({
+      id: "assignment-1",
+      actorId: "user-1",
+      returns: [
+        { assignmentLineId: "line-1", quantity: 2, notes: "Slightly damaged" },
+      ],
+    })
+    expect(mocks.revalidatePath).toHaveBeenCalledWith("/assignments")
+  })
+
+  it("surfaces use-case errors and skips revalidation on failure", async () => {
+    mocks.returnAssignmentUseCase.mockResolvedValue({
+      success: false,
+      errors: { error: ["errorConcurrent"] },
+    })
+
+    const result = await returnAssignment({
+      id: "assignment-1",
+      returns: [{ assignmentLineId: "line-1", quantity: 2 }],
+    })
+
+    expect(result).toEqual({
+      success: false as const,
+      errors: { error: ["errorConcurrent"] },
+      message: en.inventory.assignments.actions.returnFailure,
+    })
+    expect(mocks.revalidatePath).not.toHaveBeenCalled()
+  })
+
+  it("preserves the legacy full-return shortcut without returns", async () => {
+    mocks.returnAssignmentUseCase.mockResolvedValue({ success: true })
+
+    const result = await returnAssignment({ id: "assignment-1" })
+
+    expect(result.success).toBe(true)
+    expect(mocks.returnAssignmentUseCase).toHaveBeenCalledWith({
+      id: "assignment-1",
+      actorId: "user-1",
+    })
+    expect(mocks.revalidatePath).toHaveBeenCalledWith("/assignments")
+  })
+})
diff --git a/tests/unit/app/assignments/return.button.test.tsx b/tests/unit/app/assignments/return.button.test.tsx
new file mode 100644
index 0000000..4bfe39f
--- /dev/null
+++ b/tests/unit/app/assignments/return.button.test.tsx
@@ -0,0 +1,181 @@
+// @vitest-environment jsdom
+import "@testing-library/jest-dom/vitest"
+import { cleanup, render, screen } from "@testing-library/react"
+import userEvent from "@testing-library/user-event"
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest"
+
+const mocks = vi.hoisted(() => ({
+  returnAssignment: vi.fn(),
+  refresh: vi.fn(),
+  toastError: vi.fn(),
+  toastSuccess: vi.fn(),
+}))
+
+vi.mock("@/actions/assignment.actions", () => ({
+  returnAssignment: mocks.returnAssignment,
+}))
+
+vi.mock("next/navigation", () => ({
+  useRouter: () => ({ refresh: mocks.refresh }),
+}))
+
+vi.mock("sonner", () => ({
+  toast: { error: mocks.toastError, success: mocks.toastSuccess },
+}))
+
+import ReturnButton from "@/app/(dashboard)/assignments/_components/return.button"
+
+const mockCopy = {
+  title: "Devolver artículo",
+  quantity: "Cantidad",
+  quantityPlaceholder: "1",
+  notes: "Notas",
+  notesPlaceholder: "Notas opcionales",
+  submit: "Devolver",
+  cancel: "Cancelar",
+  maxQuantity: "Máximo: {max}",
+  errorConcurrent:
+    "La devolución fue modificada por otro usuario. Recarga e inténtalo de nuevo.",
+  errorGeneric: "Error al procesar la devolución",
+}
+
+describe("ReturnButton", () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+  })
+
+  afterEach(() => {
+    cleanup()
+  })
+
+  it("opens the dialog and submits a partial return", async () => {
+    mocks.returnAssignment.mockResolvedValue({
+      success: true,
+      message: "Returned",
+    })
+
+    render(
+      <ReturnButton
+        assignmentId="assignment-1"
+        ariaLabel="Return assignment"
+        assignmentLineId="line-1"
+        remainingQuantity={5}
+        copy={mockCopy}
+      />,
+    )
+
+    await userEvent.click(
+      screen.getByRole("button", { name: "Return assignment" }),
+    )
+
+    expect(screen.getByRole("dialog")).toBeInTheDocument()
+    expect(
+      screen.getByRole("heading", { name: mockCopy.title }),
+    ).toBeInTheDocument()
+
+    const quantityInput = screen.getByRole("spinbutton", {
+      name: mockCopy.quantity,
+    })
+    await userEvent.clear(quantityInput)
+    await userEvent.type(quantityInput, "2")
+
+    const notesInput = screen.getByRole("textbox", { name: mockCopy.notes })
+    await userEvent.type(notesInput, "Slightly damaged")
+
+    await userEvent.click(screen.getByRole("button", { name: mockCopy.submit }))
+
+    expect(mocks.returnAssignment).toHaveBeenCalledWith({
+      id: "assignment-1",
+      returns: [
+        {
+          assignmentLineId: "line-1",
+          quantity: 2,
+          notes: "Slightly damaged",
+        },
+      ],
+    })
+    expect(mocks.toastSuccess).toHaveBeenCalledWith("Returned")
+  })
+
+  it("disables submit and shows max quantity when quantity exceeds remaining", async () => {
+    render(
+      <ReturnButton
+        assignmentId="assignment-1"
+        ariaLabel="Return assignment"
+        assignmentLineId="line-1"
+        remainingQuantity={3}
+        copy={mockCopy}
+      />,
+    )
+
+    await userEvent.click(
+      screen.getByRole("button", { name: "Return assignment" }),
+    )
+
+    const quantityInput = screen.getByRole("spinbutton", {
+      name: mockCopy.quantity,
+    })
+    await userEvent.clear(quantityInput)
+    await userEvent.type(quantityInput, "4")
+
+    expect(screen.getByRole("button", { name: mockCopy.submit })).toBeDisabled()
+    expect(screen.getByText("Máximo: 3")).toBeInTheDocument()
+  })
+
+  it("shows concurrency error when the action returns one", async () => {
+    mocks.returnAssignment.mockResolvedValue({
+      success: false,
+      errors: { error: ["errorConcurrent"] },
+      message: "Error returning assignment",
+    })
+
+    render(
+      <ReturnButton
+        assignmentId="assignment-1"
+        ariaLabel="Return assignment"
+        assignmentLineId="line-1"
+        remainingQuantity={5}
+        copy={mockCopy}
+      />,
+    )
+
+    await userEvent.click(
+      screen.getByRole("button", { name: "Return assignment" }),
+    )
+
+    const quantityInput = screen.getByRole("spinbutton", {
+      name: mockCopy.quantity,
+    })
+    await userEvent.clear(quantityInput)
+    await userEvent.type(quantityInput, "2")
+
+    await userEvent.click(screen.getByRole("button", { name: mockCopy.submit }))
+
+    expect(screen.getByText(mockCopy.errorConcurrent)).toBeInTheDocument()
+  })
+
+  it("opens confirm dialog and submits legacy full return without quantity input", async () => {
+    mocks.returnAssignment.mockResolvedValue({
+      success: true,
+      message: "Returned",
+    })
+
+    render(
+      <ReturnButton
+        assignmentId="assignment-1"
+        ariaLabel="Return assignment"
+        copy={mockCopy}
+      />,
+    )
+
+    await userEvent.click(
+      screen.getByRole("button", { name: "Return assignment" }),
+    )
+
+    expect(screen.queryByRole("spinbutton")).not.toBeInTheDocument()
+
+    await userEvent.click(screen.getByRole("button", { name: mockCopy.submit }))
+
+    expect(mocks.returnAssignment).toHaveBeenCalledWith({ id: "assignment-1" })
+  })
+})
diff --git a/vitest.config.ts b/vitest.config.ts
index 9d71d31..5e73aaf 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -3,7 +3,7 @@ import { defineConfig } from "vitest/config"
 export default defineConfig({
   test: {
     environment: "node",
-    include: ["tests/**/*.test.ts"],
+    include: ["tests/**/*.test.ts", "tests/**/*.test.tsx"],
     watch: false,
     globals: false,
     clearMocks: true,