AI for invoicing, time and dunning: from hours to paid

Yes, AI can write an invoice that is valid in Germany and Austria, as a draft you check. It turns your hours into clear lines and writes reminders in your tone. What it must not decide: which details are mandatory, when your client is in default and, from 2027 or 2028 in Germany, the e-invoice format. Those are rules, and rules belong in your invoicing software. Below: what to hand to AI, what to keep, and two prompts to copy.

Can AI write a valid invoice in Germany and Austria?

The law only asks what is on the invoice. In Germany, § 14 (4) UStG lists ten details; for an ordinary invoice, eight of them apply: both names and addresses, your tax number or VAT ID, issue date, a unique invoice number, quantity and type of the service, the date of the service, the net amount by VAT rate, and the rate and amount of VAT (or an exemption note). Austria asks for nine under § 11 UStG, and from €10,000 gross the VAT ID of a business client as well. Up to €250 gross in Germany and €400 in Austria, a short small-amount invoice is enough. The full lists are in Invoice requirements in Germany and in Austria.

So a chatbot can write a valid invoice, if the facts are right. Where it goes wrong:

• It fills gaps instead of asking. A missing VAT ID or service date may come back as a plausible-looking invention. A wrong detail on an invoice is your problem.
• It doesn't know your situation. Small-business scheme, reverse charge, a client in another country: each changes the VAT line and the note. See Reverse charge on the invoice.
• It doesn't keep your number series and can slip on sums. Let software number and calculate.

Split the work. AI writes line text and the cover email; software numbers, calculates, checks and stores. Here is a prompt for the first half:

You help me prepare an invoice draft for a freelancer in [Germany / Austria]. Use only the facts below. If a mandatory detail is missing, list it at the end and do not invent it. Do not calculate VAT or totals; my invoicing software does that.

Facts: client [client name], period [1–30 September 2026], rate [EUR 95 per hour], my notes [paste your timesheet or notes].

Task: group the notes by project and write one invoice line per project: a clear description of the service, hours and the service period. Keep it factual, no marketing words.

Install it as a skill: Invoice writer. On first use it asks for your company details, saves them and then writes every invoice draft with all mandatory details.

Before you paste real names into any chat tool: names of people, including sole traders, are personal data. If you use a business plan, check that it comes with a data processing agreement (Art. 28 GDPR). Placeholders for names cost nothing.

How do hours become an invoice without retyping?

Track as you work, add up once a month, and let the invoice say what the law asks: the type and extent of the service and the period. For hours, that means what you did, how many hours, at what rate. In Germany the calendar month is enough as the date of the service (§ 31 (4) UStDV); in Austria you state the period (§ 11 UStG). How you round, agree with your client in the quote; round the total, not each entry. The details are in Billing hours as a freelancer.

An example: 12.5 hours of website relaunch at €95.

Here AI saves you the most time. Raw entries such as "call", "call 2", "wireframes v2" turn into three readable lines. It spots entries with no description, and it can add a cover email. What it does not do reliably is arithmetic: if your hours live in a tool, take the totals from there. Best of all is when the tool that holds your hours also creates the invoice; then nothing is copied at all.

How does AI write reminders in your tone, and when is a client in default?

AI writes the words. Whether your client is in default, the law decides:

• Germany (§ 286 BGB): default begins with a reminder sent after the due date; or without one on an agreed calendar date; or at the latest 30 days after the due date and receipt of the invoice. With consumers, the 30-day rule only applies if the invoice says so.
• Austria: default begins once the due date passes without payment (§ 907a (2) ABGB: the money must be credited on that date). A private client only has to order the transfer that day (§ 6a (2) KSchG). No reminder is needed.

Interest and the €40 flat fee (§ 288 (5) BGB, § 458 UGB) depend on the country and on whether your client is a business. Let software take the current rates; they change twice a year. See Late-payment interest. Never let a chatbot choose a rate or add fees on its own.

AI is good with tone. Show it two or three of your own emails and ask for the same voice:

Write a payment reminder in my tone. Here are two emails I wrote to clients [paste two examples] so you can match my style: short, friendly, no legalese.

Facts: invoice [RE-2026-118] of [1 September 2026], amount [EUR 2,400.00], due [15 September 2026], new deadline [29 September 2026], my bank details are in the signature.

Do not mention interest, fees or legal steps. Leave the facts above exactly as given; if something is missing, ask me.

Install it as a skill: Payment reminders in your tone. It writes the reminder, the second reminder and the final notice, with interest only where the law allows it.

For the second and third step, the wording with interest and flat fee is in Payment reminders and dunning. These stay with you: a disputed invoice (pause, call), a long-standing client (a phone call beats the second notice) and the last step before collection or court. More in AI in dunning.

What about e-invoices?

An e-invoice is structured data that software reads without retyping, in the European standard EN 16931 (for example ZUGFeRD or XRechnung). A chat answer, a Word file or a plain PDF is not one.

• Germany: since 01/01/2025 every business must be able to receive e-invoices. For domestic B2B supplies, issuing becomes mandatory from 2028 at the latest, or from 2027 if your total turnover in the previous year was above €800,000 (§ 27 (38) UStG, as of 08/10/2026). Small invoices up to €250 and invoices from small businesses stay exempt. Consumers and clients abroad are not affected. More in E-invoicing in Germany.
• Austria: no e-invoice duty between businesses; a PDF by email is fine if your client agrees. If you bill a federal body, structured e-invoices have been mandatory since 2014 (as of 08/10/2026, USP). More in E-invoicing in Austria.

Let AI write the text and let your invoicing software produce and check the file.