diff --git a/src/Controllers/InvoiceController.php b/src/Controllers/InvoiceController.php
index 6421c16..556779f 100644
--- a/src/Controllers/InvoiceController.php
+++ b/src/Controllers/InvoiceController.php
@@ -40,15 +40,26 @@ class InvoiceController
         $customers = Customer::all();
         $settings  = Setting::get();
 
+        $invoiceDefaults = [
+            'rechnungs_datum'       => date('Y-m-d'),
+            'leistungszeitraum_von' => date('Y-m-01'),
+            'leistungszeitraum_bis' => date('Y-m-t'),
+        ];
+
+        $preCustomerId = (int)($_GET['customer_id'] ?? 0);
+        if ($preCustomerId > 0) {
+            $customer = Customer::find($preCustomerId);
+            if ($customer) {
+                $invoiceDefaults['customer_id']  = $preCustomerId;
+                $invoiceDefaults['payment_days'] = $customer['payment_days'];
+            }
+        }
+
         $view = new View();
         $view->render('layout/base.php', [
             'pageTitle' => 'Neue Rechnung',
             'content'   => $this->renderView('invoices/form', [
-                'invoice'   => [
-                    'rechnungs_datum'       => date('Y-m-d'),
-                    'leistungszeitraum_von' => date('Y-m-01'),
-                    'leistungszeitraum_bis' => date('Y-m-t'),
-                ],
+                'invoice'   => $invoiceDefaults,
                 'lines'     => [],
                 'customers' => $customers,
                 'errors'    => [],
diff --git a/src/Views/invoices/form.php b/src/Views/invoices/form.php
index 066e648..ee88763 100644
--- a/src/Views/invoices/form.php
+++ b/src/Views/invoices/form.php
@@ -149,4 +149,10 @@ function updatePaymentDays(sel) {
     const days = opt.dataset.payment;
     if (days) document.getElementById('payment_days').value = days;
 }
+<?php if (!$isEdit): ?>
+document.addEventListener('DOMContentLoaded', function () {
+    const sel = document.getElementById('customer_id');
+    if (sel && sel.value) updatePaymentDays(sel);
+});
+<?php endif; ?>
 </script>
diff --git a/tests/22_RechnungZahlungszielTest.php b/tests/22_RechnungZahlungszielTest.php
new file mode 100644
index 0000000..a1ba63d
--- /dev/null
+++ b/tests/22_RechnungZahlungszielTest.php
@@ -0,0 +1,63 @@
+<?php
+
+declare(strict_types=1);
+
+use Steuerkram\Models\Customer;
+use Steuerkram\Models\Setting;
+
+class RechnungZahlungszielTest extends TestCase
+{
+    protected string $testName = 'Test 22: Rechnung – Zahlungsziel aus Kundendatensatz';
+
+    public function run(): void
+    {
+        $customerId = Customer::create([
+            'nachname'     => 'ZahlungszielTestKunde',
+            'strasse'      => 'Musterstr. 1',
+            'plz'          => '12345',
+            'ort'          => 'Musterstadt',
+            'land'         => 'DE',
+            'payment_days' => 30,
+        ]);
+
+        $customer = Customer::find($customerId);
+        $this->assert($customer !== false, 'Kunde wurde angelegt');
+        $this->assertEquals(30, (int)$customer['payment_days'], 'Kundendatensatz enthält payment_days=30');
+
+        // Simuliere Controller-Logik: customer_id per GET → payment_days vorbelegen
+        $preCustomerId = $customerId;
+        $invoiceDefaults = [
+            'rechnungs_datum'       => date('Y-m-d'),
+            'leistungszeitraum_von' => date('Y-m-01'),
+            'leistungszeitraum_bis' => date('Y-m-t'),
+        ];
+        if ($preCustomerId > 0) {
+            $c = Customer::find($preCustomerId);
+            if ($c) {
+                $invoiceDefaults['customer_id']  = $preCustomerId;
+                $invoiceDefaults['payment_days'] = $c['payment_days'];
+            }
+        }
+
+        $this->assertEquals($customerId, $invoiceDefaults['customer_id'], 'customer_id in Rechnungsdefaults gesetzt');
+        $this->assertEquals(30, (int)$invoiceDefaults['payment_days'], 'payment_days aus Kundendaten vorbelegt');
+
+        // Ohne vorausgewählten Kunden bleibt payment_days ungesetzt
+        $defaultsOhneKunde = [
+            'rechnungs_datum' => date('Y-m-d'),
+        ];
+        $this->assert(!isset($defaultsOhneKunde['payment_days']), 'payment_days ohne Kunden nicht gesetzt');
+
+        // Anderer Kunde mit abweichendem Zahlungsziel
+        $customer2Id = Customer::create([
+            'nachname'     => 'ZahlungszielTestKunde2',
+            'strasse'      => 'Testgasse 2',
+            'plz'          => '54321',
+            'ort'          => 'Testdorf',
+            'land'         => 'DE',
+            'payment_days' => 7,
+        ]);
+        $c2 = Customer::find($customer2Id);
+        $this->assertEquals(7, (int)$c2['payment_days'], 'Zweiter Kunde hat payment_days=7');
+    }
+}