diff --git a/app/Http/Controllers/Company/Invoice/InvoicesController.php b/app/Http/Controllers/Company/Invoice/InvoicesController.php
index 8ef66ff6..ee0d1f9d 100644
--- a/app/Http/Controllers/Company/Invoice/InvoicesController.php
+++ b/app/Http/Controllers/Company/Invoice/InvoicesController.php
@@ -4,6 +4,7 @@ namespace App\Http\Controllers\Company\Invoice;
 
 use App\Http\Controllers\Controller;
 use App\Http\Requests;
+use App\Http\Requests\ChangeInvoiceStatusRequest;
 use App\Http\Requests\CreateCreditNoteRequest;
 use App\Http\Requests\DeleteInvoiceRequest;
 use App\Http\Requests\SendInvoiceRequest;
@@ -238,7 +239,7 @@ class InvoicesController extends Controller
             ->setStatusCode(201);
     }
 
-    public function changeStatus(Request $request, Invoice $invoice)
+    public function changeStatus(ChangeInvoiceStatusRequest $request, Invoice $invoice)
     {
         $this->authorize('send invoice', $invoice);
 
diff --git a/app/Http/Requests/ChangeInvoiceStatusRequest.php b/app/Http/Requests/ChangeInvoiceStatusRequest.php
new file mode 100644
index 00000000..534303ed
--- /dev/null
+++ b/app/Http/Requests/ChangeInvoiceStatusRequest.php
@@ -0,0 +1,34 @@
+<?php
+
+namespace App\Http\Requests;
+
+use App\Models\Invoice;
+use Illuminate\Foundation\Http\FormRequest;
+use Illuminate\Validation\Rule;
+
+class ChangeInvoiceStatusRequest extends FormRequest
+{
+    /**
+     * Determine if the user is authorized to make this request.
+     */
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    /**
+     * Get the validation rules that apply to the request.
+     */
+    public function rules(): array
+    {
+        return [
+            'status' => [
+                'required',
+                Rule::in([
+                    Invoice::STATUS_SENT,
+                    Invoice::STATUS_COMPLETED,
+                ]),
+            ],
+        ];
+    }
+}
diff --git a/app/Services/Document/InvoiceService.php b/app/Services/Document/InvoiceService.php
index 46f9a882..f140166e 100644
--- a/app/Services/Document/InvoiceService.php
+++ b/app/Services/Document/InvoiceService.php
@@ -477,10 +477,21 @@ class InvoiceService
             $invoice->sent = true;
             $invoice->save();
         } elseif ($status == Invoice::STATUS_COMPLETED) {
-            $invoice->status = Invoice::STATUS_COMPLETED;
-            $invoice->paid_status = Invoice::STATUS_PAID;
-            $invoice->due_amount = 0;
-            $invoice->save();
+            $paid = (int) $invoice->payments()->sum('amount');
+            $credited = $this->creditNoteService->creditedTotal($invoice);
+            $outstanding = max(0, (int) $invoice->total - $paid - $credited);
+
+            if (
+                $outstanding !== 0
+                || (int) $invoice->due_amount !== 0
+                || (int) $invoice->base_due_amount !== 0
+            ) {
+                throw ValidationException::withMessages([
+                    'status' => ['invoice_must_be_settled_before_completion'],
+                ]);
+            }
+
+            $invoice->changeInvoiceStatus((int) $invoice->due_amount);
         }
     }
 }
diff --git a/lang/en.json b/lang/en.json
index 54dd6deb..e457173e 100644
--- a/lang/en.json
+++ b/lang/en.json
@@ -1854,6 +1854,7 @@
     "credit_quantity_invalid": "Enter a valid quantity greater than zero for every selected line.",
     "credit_note_cannot_be_cloned": "A credit note cannot be cloned.",
     "credit_note_cannot_be_converted_to_estimate": "A credit note cannot be converted to an estimate.",
+    "invoice_must_be_settled_before_completion": "Record a payment or create a credit note before completing this invoice.",
     "payment_amount_exceeds_invoice_due_amount": "The payment is more than the invoice's outstanding balance.",
     "payment_number_used": "The payment number has already been taken.",
     "name_already_taken": "The name has already been taken.",
diff --git a/resources/scripts/utils/error-handling.ts b/resources/scripts/utils/error-handling.ts
index dd2f285f..09f07c28 100644
--- a/resources/scripts/utils/error-handling.ts
+++ b/resources/scripts/utils/error-handling.ts
@@ -63,6 +63,8 @@ const ERROR_TRANSLATION_MAP: Record<string, string> = {
   'a_credit_note_cannot_be_cloned': 'errors.credit_note_cannot_be_cloned',
   'a_credit_note_cannot_be_converted_to_an_estimate':
     'errors.credit_note_cannot_be_converted_to_estimate',
+  'invoice_must_be_settled_before_completion':
+    'errors.invoice_must_be_settled_before_completion',
   'The estimate number has already been taken.': 'errors.estimate_number_used',
   'The payment number has already been taken.': 'errors.estimate_number_used',
   'The invoice number has already been taken.': 'errors.invoice_number_used',
diff --git a/tests/Feature/Admin/CreditNoteTest.php b/tests/Feature/Admin/CreditNoteTest.php
index ed38911e..4f1fbc42 100644
--- a/tests/Feature/Admin/CreditNoteTest.php
+++ b/tests/Feature/Admin/CreditNoteTest.php
@@ -905,18 +905,24 @@ test('no surviving row keeps a dangling related invoice reference', function ()
     expect(Invoice::find($creditNote->id)->related_invoice_id)->toBeNull();
 });
 
-test('completing an uncredited invoice still zeroes its balance', function () {
+test('completing a fully credited invoice is idempotent', function () {
     $invoice = creditableInvoice();
 
+    postJson("api/v1/invoices/{$invoice->id}/credit-note")
+        ->assertStatus(201);
+
     postJson("api/v1/invoices/{$invoice->id}/status", ['status' => Invoice::STATUS_COMPLETED])
         ->assertOk();
 
     $invoice->refresh();
 
-    // The credit-note bookkeeping must not touch the manual status change.
+    // Completion verifies the recorded credit note and does not disturb the
+    // already-settled balance.
     expect((int) $invoice->due_amount)->toBe(0)
+        ->and((int) $invoice->base_due_amount)->toBe(0)
         ->and($invoice->status)->toBe(Invoice::STATUS_COMPLETED)
-        ->and($invoice->paid_status)->toBe(Invoice::STATUS_PAID);
+        ->and($invoice->paid_status)->toBe(Invoice::STATUS_PAID)
+        ->and($invoice->payments)->toHaveCount(0);
 });
 
 test('renders a credit note pdf through the original invoice template family, not a hardcoded layout', function () {
diff --git a/tests/Feature/Admin/InvoiceTest.php b/tests/Feature/Admin/InvoiceTest.php
index a67c4add..cc33ae03 100644
--- a/tests/Feature/Admin/InvoiceTest.php
+++ b/tests/Feature/Admin/InvoiceTest.php
@@ -1,11 +1,13 @@
 <?php
 
 use App\Http\Controllers\Company\Invoice\InvoicesController;
+use App\Http\Requests\ChangeInvoiceStatusRequest;
 use App\Http\Requests\InvoicesRequest;
 use App\Mail\SendInvoiceMail;
 use App\Models\Company;
 use App\Models\Invoice;
 use App\Models\InvoiceItem;
+use App\Models\Payment;
 use App\Models\Tax;
 use App\Models\User;
 use Illuminate\Support\Facades\Artisan;
@@ -29,6 +31,23 @@ beforeEach(function () {
     );
 });
 
+function completionInvoice(int $total = 10000, ?int $dueAmount = null, ?int $baseDueAmount = null): Invoice
+{
+    $dueAmount ??= $total;
+    $baseDueAmount ??= $dueAmount;
+
+    return Invoice::factory()->create([
+        'status' => Invoice::STATUS_SENT,
+        'sent' => true,
+        'paid_status' => Invoice::STATUS_UNPAID,
+        'total' => $total,
+        'due_amount' => $dueAmount,
+        'base_total' => $total,
+        'base_due_amount' => $baseDueAmount,
+        'exchange_rate' => 1,
+    ]);
+}
+
 test('testGetInvoices', function () {
     $response = getJson('api/v1/invoices?page=1&type=OVERDUE&limit=20');
 
@@ -258,25 +277,113 @@ test('send invoice to customer', function () {
     Mail::assertSent(SendInvoiceMail::class);
 });
 
-test('invoice mark as paid', function () {
-    $invoice = Invoice::factory()->create([
-        'invoice_date' => '1988-07-18',
-        'due_date' => '1988-08-18',
+test('invoice status controller uses the change invoice status request', function () {
+    $this->assertActionUsesFormRequest(
+        InvoicesController::class,
+        'changeStatus',
+        ChangeInvoiceStatusRequest::class
+    );
+});
+
+test('cannot complete an outstanding invoice', function () {
+    $invoice = completionInvoice();
+
+    postJson("api/v1/invoices/{$invoice->id}/status", ['status' => Invoice::STATUS_COMPLETED])
+        ->assertUnprocessable()
+        ->assertJsonPath('errors.status.0', 'invoice_must_be_settled_before_completion');
+
+    $invoice->refresh();
+
+    expect((int) $invoice->due_amount)->toBe(10000)
+        ->and((int) $invoice->base_due_amount)->toBe(10000)
+        ->and($invoice->status)->toBe(Invoice::STATUS_SENT)
+        ->and($invoice->paid_status)->toBe(Invoice::STATUS_UNPAID)
+        ->and($invoice->payments)->toHaveCount(0);
+});
+
+test('cannot complete a partially paid invoice', function () {
+    $invoice = completionInvoice(10000, 5000, 5000);
+    $payment = Payment::factory()->create([
+        'company_id' => $invoice->company_id,
+        'customer_id' => $invoice->customer_id,
+        'invoice_id' => $invoice->id,
+        'amount' => 5000,
     ]);
 
-    $data = [
-        'status' => Invoice::STATUS_COMPLETED,
-    ];
+    $invoice->update(['paid_status' => Invoice::STATUS_PARTIALLY_PAID]);
 
-    $response = postJson('api/v1/invoices/'.$invoice->id.'/status', $data);
+    postJson("api/v1/invoices/{$invoice->id}/status", ['status' => Invoice::STATUS_COMPLETED])
+        ->assertUnprocessable()
+        ->assertJsonPath('errors.status.0', 'invoice_must_be_settled_before_completion');
 
-    $response
-        ->assertOk()
-        ->assertJson([
-            'success' => true,
-        ]);
+    $invoice->refresh();
 
-    $this->assertEquals(Invoice::find($invoice->id)->paid_status, Invoice::STATUS_PAID);
+    expect((int) $invoice->due_amount)->toBe(5000)
+        ->and($invoice->status)->toBe(Invoice::STATUS_SENT)
+        ->and($invoice->paid_status)->toBe(Invoice::STATUS_PARTIALLY_PAID)
+        ->and($invoice->payments->modelKeys())->toBe([$payment->id]);
+});
+
+test('cannot complete an invoice with an inconsistent zero stored due amount', function () {
+    $invoice = completionInvoice(10000, 0, 10000);
+
+    postJson("api/v1/invoices/{$invoice->id}/status", ['status' => Invoice::STATUS_COMPLETED])
+        ->assertUnprocessable()
+        ->assertJsonPath('errors.status.0', 'invoice_must_be_settled_before_completion');
+
+    $invoice->refresh();
+
+    expect((int) $invoice->due_amount)->toBe(0)
+        ->and((int) $invoice->base_due_amount)->toBe(10000)
+        ->and($invoice->status)->toBe(Invoice::STATUS_SENT)
+        ->and($invoice->payments)->toHaveCount(0);
+});
+
+test('completes a fully paid invoice idempotently without removing its payment', function () {
+    $invoice = completionInvoice(10000, 0, 0);
+    $payment = Payment::factory()->create([
+        'company_id' => $invoice->company_id,
+        'customer_id' => $invoice->customer_id,
+        'invoice_id' => $invoice->id,
+        'amount' => 10000,
+    ]);
+
+    postJson("api/v1/invoices/{$invoice->id}/status", ['status' => Invoice::STATUS_COMPLETED])
+        ->assertOk();
+    postJson("api/v1/invoices/{$invoice->id}/status", ['status' => Invoice::STATUS_COMPLETED])
+        ->assertOk();
+
+    $invoice->refresh();
+
+    expect($invoice->status)->toBe(Invoice::STATUS_COMPLETED)
+        ->and($invoice->paid_status)->toBe(Invoice::STATUS_PAID)
+        ->and($invoice->overdue)->toBe(0)
+        ->and($invoice->payments->modelKeys())->toBe([$payment->id]);
+});
+
+test('completes a zero value invoice', function () {
+    $invoice = completionInvoice(0, 0, 0);
+
+    postJson("api/v1/invoices/{$invoice->id}/status", ['status' => Invoice::STATUS_COMPLETED])
+        ->assertOk();
+
+    $invoice->refresh();
+
+    expect($invoice->status)->toBe(Invoice::STATUS_COMPLETED)
+        ->and($invoice->paid_status)->toBe(Invoice::STATUS_PAID)
+        ->and($invoice->overdue)->toBe(0);
+});
+
+test('invoice status requires a supported value', function () {
+    $invoice = completionInvoice();
+
+    postJson("api/v1/invoices/{$invoice->id}/status")
+        ->assertUnprocessable()
+        ->assertJsonValidationErrors('status');
+
+    postJson("api/v1/invoices/{$invoice->id}/status", ['status' => 'DRAFT'])
+        ->assertUnprocessable()
+        ->assertJsonValidationErrors('status');
 });
 
 test('invoice mark as sent', function () {