Turn Lexware Office into an API

What you can build on Lexware Office

• Get overdue invoices

  Build this

  Reads Überfällige in the Belege section: the invoices past their due date, with customer, amounts and due date.

  Takes

  Nothing

  Returns

  Invoice number, customer, invoice date, due date, total, open amount, status

• Get open vouchers

  Build this

  Reads Offene Belege in the Belege section: each open voucher's contact, due date and the amount still open, filtered by voucher type if you give one.

  Takes

  Voucher type (optional), due before (optional)

  Returns

  Voucher number, type, contact, voucher date, due date, total, open amount

• Record a payment on a voucher

  Build this

  Selects a voucher in the Belegliste, chooses Zahlung erfassen and enters the amount, payment date and payment type Privat or Kasse.

  Takes

  Voucher number, amount, payment date, payment type

  Returns

  Voucher number, amount recorded, open amount, status

• Prepare a Mahnung for an invoice

  Build this

  Opens an overdue invoice, chooses Mahnen under Weitere Aktionen, sets the title and text, adds a Mahngebühr if given and saves it without sending.

  Takes

  Invoice number, title, dunning fee (optional)

  Returns

  Invoice number, Mahnung title, fee, saved date

Automate Lexware Office with a workflow

A workflow runs your methods as one process, with nobody at the screen. A schedule or a call from your own system starts it, the steps run in order, and the result arrives in your inbox or in the systems you already use.

• Überfällige invoices, every weekday morning

  Build this

  Starts

  Every weekday at 8:00

  Step 1

  Get the invoices listed under Überfällige in Belege.

  Step 2

  Keep the ones with an open amount above zero.

  Step 3

  Email you the list sorted by due date, with the total still open.

• Purchase invoices due this week, to your payment run

  Build this

  Starts

  Every Monday at 7:00

  Step 1

  Get the Offene Belege of type purchase invoice due within the next 7 days.

  Step 2

  Stop when there are none.

  Step 3

  Post voucher number, supplier, due date and open amount to your ERP or banking webhook.

• New overdue invoices, with a Mahnung ready to review

  Build this

  Starts

  Every weekday at 9:00

  Step 1

  Get the invoices listed under Überfällige in Belege.

  Step 2

  Keep the ones that were not overdue on the last run.

  Step 3

  For each one, prepare a Mahnung titled Zahlungserinnerung and save it without sending.

  Step 4

  Email you the invoices, so you can check each Mahnung and send it yourself.

You can run a workflow by hand on every plan, Free included. Schedules and webhook triggers come with the paid plans, from $29 a month.

Once built, one call from your code

Once you have built a method, it is one POST to /v1/runs from your backend, or a tool an AI agent calls over MCP. This is what “Get overdue invoices” would look like.

curl https://api.fous.com/v1/runs \
  -H "Authorization: Bearer $FOUS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "method": "app.lexware.de/get_overdue_invoices",
    "input": {}
  }'

{
  "status": "completed",
  "data": {
    "invoices": [
      {
        "invoice_number": "RE-2026-0418",
        "customer": "Northwind GmbH",
        "invoice_date": "2026-10-08",
        "due_date": "2026-10-22",
        "total": 2380,
        "open_amount": 2380,
        "currency": "EUR",
        "status": "overdue"
      }
    ],
    "count": 1,
    "open_total": 2970
  }
}

An example with invented data, not a ready-made endpoint. The method exists in your organization once you build it, and its name, inputs and fields follow your request.

Lexware Office’s own options

Lexware Office has a Public API for customers on version XL, listed at 32,90 € a month before VAT. It covers contacts, invoices, vouchers, the voucher list, payments and files, at 2 requests per second, with a key you create in the Public API Verwaltung. Versions S, M and L do not include it. Bookkeeping data also goes to the tax adviser through DATEV exports.

Read on November 12, 2026: Alles rund um Public API, Lexware Public API documentation, Welche DATEV-Exporte bietet Lexware Office an?.

Signing in to Lexware Office

Lexware Office signs you in with your email address and password. Anmeldung in zwei Schritten (two-step sign-in) is optional: a push message in the Lexware App or a code from an authenticator app. With Fous you sign in once through Lexware Office's own sign-in with your own account: the login is sealed with AES-256, an approval in the app on your phone is handled, and an expired session is signed back in.

Worth knowing

One company per account

A Lexware Office account holds one company or trade. A second company needs its own account with its own email address, so a method works on the company of the account it signs in with.

Overdue is a list of its own

Overdue vouchers have their own entry, Überfällige, next to Offene Belege in Belege, and are marked orange in the lists. A Mahnung starts from the invoice under Weitere Aktionen, Mahnen.

Roles limit what a method can do

Users with the role Belege, Kontakte & Produkte cannot delete payments. A method reads and does only what the account it signs in with is allowed to.

The API's voucher list needs a status

In the Public API, the voucher list endpoint requires both a voucher type and a voucher status filter, and the key's permissions cannot be changed later: you create a new key instead.

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