diff --git a/src/use-cases/assignment.use-cases.ts b/src/use-cases/assignment.use-cases.ts
index f619f54..e9b5cd3 100644
--- a/src/use-cases/assignment.use-cases.ts
+++ b/src/use-cases/assignment.use-cases.ts
@@ -244,8 +244,17 @@ export async function updateAssignmentUseCase(
       return updateAssignmentError({ id: ["Assignment not found"] })
     }
 
+    if (
+      assignment.status === "RETURNED" ||
+      assignment.status === "CANCELLED"
+    ) {
+      return updateAssignmentError({ id: ["Assignment is closed"] })
+    }
+
+    let item = null
+
     if (itemId) {
-      const item = await ItemService.findById(itemId, tx)
+      item = await ItemService.findById(itemId, tx)
 
       if (!item) {
         return updateAssignmentError({ itemId: ["Item not found"] })
@@ -308,6 +317,22 @@ export async function updateAssignmentUseCase(
         tx,
       )
     } else {
+      if (item && assignment.quantity !== quantity) {
+        const stockDelta = quantity - (assignment.quantity ?? 0)
+        const adjustmentItemId = item.id
+        await ItemService.updateStock(adjustmentItemId, -stockDelta, tx)
+        await MovementService.create(
+          {
+            type: "ADJUSTMENT",
+            itemId: adjustmentItemId,
+            quantity: Math.abs(stockDelta),
+            stockDeltaSign: stockDelta < 0 ? 1 : -1,
+            userId: actorId,
+          },
+          tx,
+        )
+      }
+
       await AssignmentService.update(
         id,
         {
diff --git a/tests/integration/use-cases/assignment.use-cases.test.ts b/tests/integration/use-cases/assignment.use-cases.test.ts
index d7b4b49..e35ca97 100644
--- a/tests/integration/use-cases/assignment.use-cases.test.ts
+++ b/tests/integration/use-cases/assignment.use-cases.test.ts
@@ -408,4 +408,240 @@ describe("assignment use-cases", () => {
 
     expect(await prisma.inventoryMovement.count()).toBe(2)
   })
+
+  it("writes an ADJUSTMENT movement with stockDelta=+1 when quantity decreases on an OPEN assignment", async () => {
+    const actor = await createTestUser(prisma)
+    const person = await createTestPerson(prisma)
+    const item = await createTestItem(prisma, { stock: 5 })
+
+    const created = await createAssignmentUseCase({
+      actorId: actor.id,
+      personId: person.id,
+      lines: [{ itemId: item.id, quantity: 2 }],
+    })
+    expect(created.success).toBe(true)
+    if (!created.success) throw new Error("Expected assignment creation success")
+
+    const updated = await updateAssignmentUseCase({
+      actorId: actor.id,
+      id: created.assignmentId,
+      personId: person.id,
+      lines: [{ itemId: item.id, quantity: 1 }],
+    })
+    expect(updated.success).toBe(true)
+    if (!updated.success) throw new Error("Expected assignment update success")
+
+    const [updatedItem, movements] = await Promise.all([
+      prisma.item.findUniqueOrThrow({ where: { id: item.id } }),
+      prisma.inventoryMovement.findMany({
+        include: { stockLines: true },
+        orderBy: [{ createdAt: "asc" }, { id: "asc" }],
+      }),
+    ])
+
+    expect(updatedItem.stock).toBe(4)
+    const adjustment = movements.find((movement) => movement.type === "ADJUSTMENT")
+    expect(adjustment).toBeDefined()
+    expect(adjustment).toMatchObject({
+      type: "ADJUSTMENT",
+      reason: "INVENTORY_CORRECTION",
+    })
+    expect(adjustment?.stockLines[0]).toMatchObject({
+      itemId: item.id,
+      stockDelta: 1,
+      previousStock: 3,
+      newStock: 4,
+    })
+  })
+
+  it("writes an ADJUSTMENT movement with stockDelta=+2 when quantity decreases on a PARTIALLY_RETURNED assignment", async () => {
+    const actor = await createTestUser(prisma)
+    const person = await createTestPerson(prisma)
+    const item = await createTestItem(prisma, { stock: 5 })
+
+    const created = await createAssignmentUseCase({
+      actorId: actor.id,
+      personId: person.id,
+      lines: [{ itemId: item.id, quantity: 3 }],
+    })
+    expect(created.success).toBe(true)
+    if (!created.success) throw new Error("Expected assignment creation success")
+
+    const createdAssignment = await prisma.assignment.findUniqueOrThrow({
+      where: { id: created.assignmentId },
+      include: { stockLines: true },
+    })
+
+    const partialReturn = await returnAssignmentUseCase({
+      id: created.assignmentId,
+      actorId: actor.id,
+      returns: [
+        {
+          assignmentLineId: createdAssignment.stockLines[0].id,
+          quantity: 1,
+        },
+      ],
+    })
+    expect(partialReturn).toEqual({ success: true })
+
+    const updated = await updateAssignmentUseCase({
+      actorId: actor.id,
+      id: created.assignmentId,
+      personId: person.id,
+      lines: [{ itemId: item.id, quantity: 1 }],
+    })
+    expect(updated.success).toBe(true)
+    if (!updated.success) throw new Error("Expected assignment update success")
+
+    const [updatedItem, movements] = await Promise.all([
+      prisma.item.findUniqueOrThrow({ where: { id: item.id } }),
+      prisma.inventoryMovement.findMany({
+        include: { stockLines: true },
+        orderBy: [{ createdAt: "asc" }, { id: "asc" }],
+      }),
+    ])
+
+    expect(updatedItem.stock).toBe(5)
+    const adjustment = movements.find((movement) => movement.type === "ADJUSTMENT")
+    expect(adjustment).toBeDefined()
+    expect(adjustment).toMatchObject({
+      type: "ADJUSTMENT",
+      reason: "INVENTORY_CORRECTION",
+    })
+    expect(adjustment?.stockLines[0]).toMatchObject({
+      itemId: item.id,
+      stockDelta: 2,
+      previousStock: 3,
+      newStock: 5,
+    })
+  })
+
+  it("writes an ADJUSTMENT movement with stockDelta=-1 when quantity increases on an OPEN assignment", async () => {
+    const actor = await createTestUser(prisma)
+    const person = await createTestPerson(prisma)
+    const item = await createTestItem(prisma, { stock: 5 })
+
+    const created = await createAssignmentUseCase({
+      actorId: actor.id,
+      personId: person.id,
+      lines: [{ itemId: item.id, quantity: 2 }],
+    })
+    expect(created.success).toBe(true)
+    if (!created.success) throw new Error("Expected assignment creation success")
+
+    const updated = await updateAssignmentUseCase({
+      actorId: actor.id,
+      id: created.assignmentId,
+      personId: person.id,
+      lines: [{ itemId: item.id, quantity: 3 }],
+    })
+    expect(updated.success).toBe(true)
+    if (!updated.success) throw new Error("Expected assignment update success")
+
+    const [updatedItem, movements] = await Promise.all([
+      prisma.item.findUniqueOrThrow({ where: { id: item.id } }),
+      prisma.inventoryMovement.findMany({
+        include: { stockLines: true },
+        orderBy: [{ createdAt: "asc" }, { id: "asc" }],
+      }),
+    ])
+
+    expect(updatedItem.stock).toBe(2)
+    const adjustment = movements.find((movement) => movement.type === "ADJUSTMENT")
+    expect(adjustment).toBeDefined()
+    expect(adjustment).toMatchObject({
+      type: "ADJUSTMENT",
+      reason: "INVENTORY_CORRECTION",
+    })
+    expect(adjustment?.stockLines[0]).toMatchObject({
+      itemId: item.id,
+      stockDelta: -1,
+      previousStock: 3,
+      newStock: 2,
+    })
+  })
+
+  it("rejects a quantity change on a RETURNED assignment and writes no movement", async () => {
+    const actor = await createTestUser(prisma)
+    const person = await createTestPerson(prisma)
+    const item = await createTestItem(prisma, { stock: 5 })
+
+    const created = await createAssignmentUseCase({
+      actorId: actor.id,
+      personId: person.id,
+      lines: [{ itemId: item.id, quantity: 2 }],
+    })
+    expect(created.success).toBe(true)
+    if (!created.success) throw new Error("Expected assignment creation success")
+
+    const returned = await returnAssignmentUseCase({
+      id: created.assignmentId,
+      actorId: actor.id,
+    })
+    expect(returned).toEqual({ success: true })
+
+    const movementsBefore = await prisma.inventoryMovement.count()
+
+    const updated = await updateAssignmentUseCase({
+      actorId: actor.id,
+      id: created.assignmentId,
+      personId: person.id,
+      lines: [{ itemId: item.id, quantity: 3 }],
+    })
+
+    expect(updated).toEqual({
+      success: false,
+      errors: { id: ["Assignment is closed"] },
+    })
+
+    const [assignment, movements] = await Promise.all([
+      prisma.assignment.findUniqueOrThrow({
+        where: { id: created.assignmentId },
+        include: { stockLines: true },
+      }),
+      prisma.inventoryMovement.findMany({
+        include: { stockLines: true },
+      }),
+    ])
+
+    expect(assignment.stockLines[0].quantity).toBe(2)
+    expect(movements).toHaveLength(movementsBefore)
+    expect(movements.some((movement) => movement.type === "ADJUSTMENT")).toBe(false)
+  })
+
+  it("writes no movement when quantity is unchanged on an OPEN assignment", async () => {
+    const actor = await createTestUser(prisma)
+    const person = await createTestPerson(prisma)
+    const item = await createTestItem(prisma, { stock: 5 })
+
+    const created = await createAssignmentUseCase({
+      actorId: actor.id,
+      personId: person.id,
+      lines: [{ itemId: item.id, quantity: 2 }],
+    })
+    expect(created.success).toBe(true)
+    if (!created.success) throw new Error("Expected assignment creation success")
+
+    const movementsBefore = await prisma.inventoryMovement.count()
+
+    const updated = await updateAssignmentUseCase({
+      actorId: actor.id,
+      id: created.assignmentId,
+      personId: person.id,
+      lines: [{ itemId: item.id, quantity: 2 }],
+    })
+    expect(updated).toEqual({ success: true })
+
+    const [updatedItem, movements] = await Promise.all([
+      prisma.item.findUniqueOrThrow({ where: { id: item.id } }),
+      prisma.inventoryMovement.findMany({
+        include: { stockLines: true },
+        orderBy: [{ createdAt: "asc" }, { id: "asc" }],
+      }),
+    ])
+
+    expect(updatedItem.stock).toBe(3)
+    expect(movements).toHaveLength(movementsBefore)
+    expect(movements.some((movement) => movement.type === "ADJUSTMENT")).toBe(false)
+  })
 })