diff --git a/backend/__tests__/integration/discountLineItems.test.js b/backend/__tests__/integration/discountLineItems.test.js
new file mode 100644
index 00000000..6b222d36
--- /dev/null
+++ b/backend/__tests__/integration/discountLineItems.test.js
@@ -0,0 +1,79 @@
+/**
+ * Negative line items (Rabatt / manual discount lines) are accepted
+ * end-to-end as long as the resulting total stays ≥ 0. When the
+ * discount would drive the total negative, the service rejects with
+ * a clear, code-tagged error so the admin is steered to Storno for
+ * credit-note workflows.
+ *
+ * Touches the actual createInvoice / createQuote service paths so a
+ * future change to either computeTotals or the guard fires this test.
+ */
+
+const { bootCrmDb, seedMinimal } = require('./helpers/crmDb');
+
+// Service-level CRM calls cold-require heavy modules (pdfService,
+// nodemailer, etc.) on first use; the global 5 s per-test budget is
+// too tight for that. Bump it for this file only.
+jest.setTimeout(30000);
+
+describe('discount line items (negative unit_price_minor)', () => {
+  let db;
+  let cleanup;
+  let adminId;
+  let customerId;
+  let invoiceService;
+
+  beforeAll(async () => {
+    ({ db, cleanup } = await bootCrmDb());
+    ({ adminId, customerId } = await seedMinimal(db));
+    invoiceService = require('../../src/services/invoiceService');
+  }, 120000);
+
+  afterAll(async () => {
+    if (cleanup) await cleanup();
+  });
+
+  // Quote-side coverage of the symmetric validator + guard is
+  // deliberately omitted: createQuote's init path takes ~30 s under
+  // this harness (something in pdfService / emailProcessor cold-
+  // require), which would push the suite well past CI's per-test
+  // budget. The shape of the guard is identical to the invoice one
+  // covered below; a future change to extract the slow init or to
+  // stub it for tests should re-enable a parallel quote test.
+
+  describe('invoices', () => {
+    it('accepts a negative-price line and computes the net correctly', async () => {
+      const { invoiceIds } = await invoiceService.createInvoice({
+        customerAccountId: customerId,
+        currency: 'CHF',
+        vatRate: 0,
+        lineItems: [
+          { position: 1, quantity: 1, description: 'Photo service', unit_price_minor: 20000, discount_percent: 0 },
+          { position: 2, quantity: 1, description: 'Treuerabatt',   unit_price_minor: -5000, discount_percent: 0 },
+        ],
+      }, adminId);
+
+      expect(Array.isArray(invoiceIds)).toBe(true);
+      expect(invoiceIds.length).toBe(1);
+
+      const row = await db('invoices').where({ id: invoiceIds[0] }).first();
+      expect(row.net_amount_minor).toBe(15000);
+      expect(row.total_amount_minor).toBe(15000);
+    });
+
+    it('rejects when the discount drives the total negative', async () => {
+      await expect(invoiceService.createInvoice({
+        customerAccountId: customerId,
+        currency: 'CHF',
+        vatRate: 0,
+        lineItems: [
+          { position: 1, quantity: 1, description: 'Photo service', unit_price_minor: 10000, discount_percent: 0 },
+          { position: 2, quantity: 1, description: 'Übergroßer Rabatt', unit_price_minor: -50000, discount_percent: 0 },
+        ],
+      }, adminId)).rejects.toMatchObject({
+        code: 'INVOICE_TOTAL_NEGATIVE',
+        statusCode: 400,
+      });
+    });
+  });
+});
diff --git a/backend/src/routes/adminInvoices.js b/backend/src/routes/adminInvoices.js
index 51ce2334..c8f4e7c5 100644
--- a/backend/src/routes/adminInvoices.js
+++ b/backend/src/routes/adminInvoices.js
@@ -247,7 +247,11 @@ const INVOICE_BODY_VALIDATORS = [
   body('lineItems').optional({ values: 'falsy' }).isArray(),
   body('lineItems.*.description').optional({ values: 'falsy' }).isString().isLength({ min: 1, max: 1000 }),
   body('lineItems.*.quantity').optional({ values: 'falsy' }).isFloat({ min: 0 }),
-  body('lineItems.*.unitPriceMinor').optional({ values: 'falsy' }).isInt({ min: 0 }),
+  // Negative unit prices are allowed so admins can add manual
+  // discount / Rabatt lines (e.g. "Treuerabatt -50,00 €"). The
+  // service-layer total guard rejects invoices whose net goes below
+  // zero — for credit notes, use Storno instead.
+  body('lineItems.*.unitPriceMinor').optional({ values: 'falsy' }).isInt(),
   body('lineItems.*.discountPercent').optional({ values: 'falsy' }).isFloat({ min: 0, max: 100 }),
   // Migration 119: sub-item + details support. Cross-row constraints
   // (parent must exist, max 1 level deep) are enforced by the service
@@ -663,6 +667,16 @@ router.put(
       updates.shipping_amount_minor = shipping;
       updates.total_amount_minor = net + vatAmount + shipping;
 
+      // Negative line items (Rabatt) are allowed, but the resulting
+      // invoice total must not go below zero. Credit notes belong in
+      // the Storno path, not in regular invoice edits.
+      if (updates.total_amount_minor < 0) {
+        return res.status(400).json({
+          error: 'Invoice total cannot be negative. To issue a credit note, cancel the original invoice with Storno.',
+          code: 'INVOICE_TOTAL_NEGATIVE',
+        });
+      }
+
       const quoteService = require('../services/quoteService');
       const { validateLineItemHierarchy, insertLineItemsHierarchical } = quoteService._internal;
       await db.transaction(async (trx) => {
diff --git a/backend/src/routes/adminQuotes.js b/backend/src/routes/adminQuotes.js
index 4900c280..417d6dda 100644
--- a/backend/src/routes/adminQuotes.js
+++ b/backend/src/routes/adminQuotes.js
@@ -335,7 +335,11 @@ const QUOTE_BODY_VALIDATORS = [
   body('lineItems').optional({ values: 'falsy' }).isArray(),
   body('lineItems.*.description').optional({ values: 'falsy' }).isString().isLength({ min: 1, max: 1000 }),
   body('lineItems.*.quantity').optional({ values: 'falsy' }).isFloat({ min: 0 }),
-  body('lineItems.*.unitPriceMinor').optional({ values: 'falsy' }).isInt({ min: 0 }),
+  // Negative unit prices are allowed so admins can add manual
+  // discount / Rabatt lines (e.g. "Treuerabatt -50,00 €"). The
+  // service-layer total guard rejects quotes whose net goes below
+  // zero — see quoteService.
+  body('lineItems.*.unitPriceMinor').optional({ values: 'falsy' }).isInt(),
   body('lineItems.*.discountPercent').optional({ values: 'falsy' }).isFloat({ min: 0, max: 100 }),
   // Migration 119: sub-item + details support. Cross-row constraints
   // (parent must exist, max 1 level deep) are enforced by the service
diff --git a/backend/src/services/invoiceService.js b/backend/src/services/invoiceService.js
index 2f0f7266..e0b5879b 100644
--- a/backend/src/services/invoiceService.js
+++ b/backend/src/services/invoiceService.js
@@ -696,6 +696,18 @@ async function createInvoice(payload, adminId, trx = db) {
   const shippingMinor = ensureInt(payload.shippingAmountMinor);
   const totalMinor = netMinor + vatMinor + shippingMinor;
 
+  // Negative line items (Rabatt) are allowed, but the resulting
+  // invoice total must not go below zero. Credit notes belong in
+  // the Storno path (createStorno), which mints a separate
+  // kind='storno' record with cancels_invoice_id set.
+  if (totalMinor < 0) {
+    throw new AppError(
+      'Invoice total cannot be negative. To issue a credit note, cancel the original invoice with Storno.',
+      400,
+      'INVOICE_TOTAL_NEGATIVE',
+    );
+  }
+
   const bank = await businessProfileService.resolveBankAccountForCurrency(currency, payload.businessBankAccountId);
 
   // Snapshot the selected payment-term template (net days / Skonto /
diff --git a/backend/src/services/quoteService.js b/backend/src/services/quoteService.js
index 9ab6c64e..b93c7528 100644
--- a/backend/src/services/quoteService.js
+++ b/backend/src/services/quoteService.js
@@ -476,6 +476,17 @@ async function createQuote(payload, adminId) {
     payload.shippingAmountMinor
   );
 
+  // Negative line items (Rabatt) are allowed, but the resulting
+  // quote total must not go below zero — a quote represents an
+  // offer of value, not a credit note.
+  if (totals.totalAmountMinor < 0) {
+    throw new AppError(
+      'Quote total cannot be negative. Reduce the discount amount.',
+      400,
+      'QUOTE_TOTAL_NEGATIVE',
+    );
+  }
+
   // Resolve bank account for the chosen currency.
   const bank = await businessProfileService.resolveBankAccountForCurrency(currency, payload.businessBankAccountId);
 
@@ -586,6 +597,16 @@ async function updateQuote(id, payload, adminId) {
     payload.shippingAmountMinor ?? existing.shipping_amount_minor
   );
 
+  // Negative line items (Rabatt) are allowed, but the resulting
+  // quote total must not go below zero. See createQuote.
+  if (totals.totalAmountMinor < 0) {
+    throw new AppError(
+      'Quote total cannot be negative. Reduce the discount amount.',
+      400,
+      'QUOTE_TOTAL_NEGATIVE',
+    );
+  }
+
   return await db.transaction(async (trx) => {
     const updates = {
       updated_at: new Date(),