app/Domains/CostUnit/Controllers/ExportController.php

@@ -93,7 +93,7 @@ class ExportController extends CommonController {
                     'amount' => $invoice->amount,
                     'recipient_name' => $invoice->contact_bank_owner,
                     'recipient_iban' => $invoice->contact_bank_iban,
-                    'payment_purpose' => $invoice->payment_purpose ?? 'Auslagenerstattung Rechnungsnummer ' . $invoice->invoice_number,
+                    'payment_purpose' => $invoice->paymentPurposeText(),
                 ]);
             }
         }

app/Models/Invoice.php

@@ -71,6 +71,26 @@ class Invoice extends InstancedModel
         'denied_reason',
     ];
 
+    /**
+     * Der Verwendungszweck für Überweisung, Buchungstext und Anzeige.
+     *
+     * Der Freitext gewinnt, wenn einer erfasst wurde. Sonst benennt der Text den Vorgang: eine
+     * Beitragserstattung ist keine Auslage des Teilis, sondern die Rücknahme seiner Zahlung -- auf dem
+     * Kontoauszug muss der Unterschied erkennbar sein. Genannt wird die Abrechnungsnummer, und zwar als
+     * Belegnummer: unter "Rechnungsnummer" gibt es sie nirgends.
+     */
+    public function paymentPurposeText() : string {
+        if ($this->payment_purpose !== null) {
+            return $this->payment_purpose;
+        }
+
+        $subject = $this->type === InvoiceType::INVOICE_TYPE_PARTICIPATION_REFUND
+            ? 'Beitragserstattung'
+            : 'Auslagenerstattung';
+
+        return $subject . ' Belegnummer ' . $this->invoice_number;
+    }
+
     public function costUnit() : BelongsTo{
         return $this->belongsTo(CostUnit::class);
     }

app/Resources/InvoiceResource.php

@@ -39,7 +39,7 @@ class InvoiceResource {
         $returnData['id'] = $this->invoice->id;
         $returnData['donation'] = $this->invoice->donation;
         $returnData['externalPayment'] = null !== $this->invoice->payment_purpose;
-        $returnData['paymentPurpose'] = $this->invoice->payment_purpose ?? 'Auslagenerstattung Rechnungsnummer ' . $returnData['invoiceNumber'];
+        $returnData['paymentPurpose'] = $this->invoice->paymentPurposeText();
         $returnData['accountOwner'] = $this->invoice->contact_bank_owner ?? '--';
         $returnData['accountIban'] = $this->invoice->contact_bank_iban ?? '--';
         $returnData['status'] = $this->invoice->status;

tests/Feature/RefundInvoiceTest.php

@@ -70,6 +70,16 @@ class RefundInvoiceTest extends TestCase
         DB::table('participation_fee_types')->insert(['slug' => 'fixed', 'name' => 'Fix']);
         DB::table('cost_unit_types')->insert(['slug' => CostUnitType::COST_UNIT_TYPE_EVENT, 'name' => 'Veranstaltung']);
         DB::table('invoice_status')->insert(['slug' => InvoiceStatus::INVOICE_STATUS_NEW]);
+
+        // Ein gewöhnlicher Aufwandstyp zum Vergleich; die Beitragserstattung bringt die Migration mit.
+        DB::table('invoice_types')->insert([
+            'slug' => InvoiceType::INVOICE_TYPE_TRAVELLING,
+            'name' => 'Fahrtkosten',
+            'sort_order' => 1,
+            'selectable' => true,
+            'counts_as_expense' => true,
+        ]);
+
         PaymentMethod::create(['slug' => PaymentMethod::PAYMENT_ACCOUNT_TRANSACTION]);
         EfzStatus::create(['slug' => EfzStatus::EFZ_STATUS_NOT_REQUIRED, 'name' => 'Nicht erforderlich']);
 
@@ -433,4 +443,55 @@ class RefundInvoiceTest extends TestCase
         $this->assertStringContainsString('300,00 Euro', Invoice::first()->comment);
         $this->assertEqualsWithDelta(80.0, $refund->participant->fresh()->amount_paid->getAmount(), 0.001);
     }
+
+    /*
+    |--------------------------------------------------------------------------
+    | Der Verwendungszweck der Überweisung
+    |--------------------------------------------------------------------------
+    */
+
+    public function test_the_payment_purpose_of_a_refund_names_the_refund(): void
+    {
+        $this->runRefund();
+
+        $invoice = Invoice::first();
+
+        // Auf dem Kontoauszug des Teilis muss der Vorgang stehen, den es gab: Er hatte keine Auslage,
+        // er bekommt seinen Beitrag zurück.
+        $this->assertSame(
+            'Beitragserstattung Belegnummer ' . $invoice->invoice_number,
+            $invoice->paymentPurposeText()
+        );
+    }
+
+    public function test_an_ordinary_invoice_keeps_the_expense_wording(): void
+    {
+        $invoice = Invoice::create([
+            'tenant' => $this->tenant->slug,
+            'cost_unit_id' => $this->makeCostUnit()->id,
+            'invoice_number' => '2026-0042',
+            'status' => InvoiceStatus::INVOICE_STATUS_NEW,
+            'type' => InvoiceType::INVOICE_TYPE_TRAVELLING,
+            'contact_name' => 'Mika Muster',
+            'amount' => 42.0,
+        ]);
+
+        $this->assertSame('Auslagenerstattung Belegnummer 2026-0042', $invoice->paymentPurposeText());
+    }
+
+    public function test_a_free_text_purpose_wins(): void
+    {
+        $invoice = Invoice::create([
+            'tenant' => $this->tenant->slug,
+            'cost_unit_id' => $this->makeCostUnit()->id,
+            'invoice_number' => '2026-0043',
+            'status' => InvoiceStatus::INVOICE_STATUS_NEW,
+            'type' => InvoiceType::INVOICE_TYPE_TRAVELLING,
+            'contact_name' => 'Mika Muster',
+            'amount' => 42.0,
+            'payment_purpose' => 'Sommerlager',
+        ]);
+
+        $this->assertSame('Sommerlager', $invoice->paymentPurposeText());
+    }
 }