diff --git a/backend/src/utils/functions.ts b/backend/src/utils/functions.ts
index 3fa2e59..764fa48 100644
--- a/backend/src/utils/functions.ts
+++ b/backend/src/utils/functions.ts
@@ -5,7 +5,7 @@ import bwipjs from "bwip-js"
 import Sharp from "sharp"
 
 import { tenants } from "../../db/schema"
-import { eq } from "drizzle-orm"
+import { eq, sql } from "drizzle-orm"
 
 export const useNextNumberRangeNumber = async (
     server: FastifyInstance,
@@ -19,47 +19,53 @@ export const useNextNumberRangeNumber = async (
         cancellationInvoices: "invoices",
     }
 
-    const [tenant] = await server.db
-        .select()
-        .from(tenants)
-        .where(eq(tenants.id, tenantId))
+    return server.db.transaction(async (tx) => {
+        // Serialisiert Nummernvergaben dieses Mandanten auch ueber mehrere
+        // Backend-Prozesse hinweg, bis die Transaktion abgeschlossen ist.
+        await tx.execute(sql`select pg_advisory_xact_lock(${tenantId})`)
 
-    if (!tenant) {
-        throw new Error(`Tenant ${tenantId} not found`)
-    }
+        const [tenant] = await tx
+            .select()
+            .from(tenants)
+            .where(eq(tenants.id, tenantId))
 
-    const numberRanges = tenant.numberRanges || {}
+        if (!tenant) {
+            throw new Error(`Tenant ${tenantId} not found`)
+        }
 
-    const resolvedNumberRange = numberRanges[numberRange]
-        ? numberRange
-        : numberRangeFallbacks[numberRange]
+        const numberRanges = tenant.numberRanges || {}
 
-    if (!resolvedNumberRange || !numberRanges[resolvedNumberRange]) {
-        throw new Error(`Number range '${numberRange}' not found`)
-    }
+        const resolvedNumberRange = numberRanges[numberRange]
+            ? numberRange
+            : numberRangeFallbacks[numberRange]
 
-    const current = numberRanges[resolvedNumberRange]
+        if (!resolvedNumberRange || !numberRanges[resolvedNumberRange]) {
+            throw new Error(`Number range '${numberRange}' not found`)
+        }
 
-    const usedNumber =
-        (current.prefix || "") +
-        current.nextNumber +
-        (current.suffix || "")
+        const current = numberRanges[resolvedNumberRange]
 
-    const updatedRanges = {
-        // @ts-ignore
-        ...numberRanges,
-        [resolvedNumberRange]: {
-            ...current,
-            nextNumber: current.nextNumber + 1,
-        },
-    }
+        const usedNumber =
+            (current.prefix || "") +
+            current.nextNumber +
+            (current.suffix || "")
 
-    await server.db
-        .update(tenants)
-        .set({ numberRanges: updatedRanges })
-        .where(eq(tenants.id, tenantId))
+        const updatedRanges = {
+            // @ts-ignore
+            ...numberRanges,
+            [resolvedNumberRange]: {
+                ...current,
+                nextNumber: current.nextNumber + 1,
+            },
+        }
 
-    return { usedNumber }
+        await tx
+            .update(tenants)
+            .set({ numberRanges: updatedRanges })
+            .where(eq(tenants.id, tenantId))
+
+        return { usedNumber }
+    })
 }
 
 export async function encodeBase64ToNiimbot(base64Png: string, printDirection: "top" | "left" = "top") {
diff --git a/frontend/pages/createDocument/edit/[[id]].vue b/frontend/pages/createDocument/edit/[[id]].vue
index bec580e..282d77c 100644
--- a/frontend/pages/createDocument/edit/[[id]].vue
+++ b/frontend/pages/createDocument/edit/[[id]].vue
@@ -1761,7 +1761,7 @@ const saveDocument = async (state, resetup = false) => {
   itemInfo.value.state = state
   normalizeCustomSurchargePercentage()
 
-  if (state !== "Entwurf") {
+  if (state !== "Entwurf" && !itemInfo.value.documentNumber) {
     console.log("???")
 
     const type = itemInfo.value.type