This is a property of how VAT works. In the common scenario, the seller is effectively "tax administrator" for the buyer (much like employer pays payroll taxes on behalf of employee), therefore an invoice triggers two independent payables: 1. Seller to taxman 2. Buyer to seller

Neither seller cares how you handle your payments to the taxman, nor taxman cares about customer payments.

The second part regarding credit/debit invoices is again a property of "append-only double-entry bookkeeping". You fix wrong credit/debit entry with reversed debit/credit entry.

To spell that out: if you made a mistake in an invoice or negotiated a new total or an extra item (or had to drop an item) after creating the invoice, you now need to first create a correction invoice cancelling out the invoice you created and then create a new invoice. All three documents must have different numbers. Oh, and invoice numbers must be sequential and continuous.

Many years ago, my dad wrote an accounting system for the company he worked at to automate some of this. He spent ages trying to convince the tax authorities that the database was sufficient, and that there was no point in printing out an extra copy of every invoice and gluing it into a book. The idea is it'd make it harder for you to retroactively manipulate invoices.

They eventually saw the light, but it was a long slog.

In the recent years Germany has started allowing companies to no longer issue print invoices. But you still need to keep a copy of every generated invoice.

Initially "digital invoice" meant that you had to get the invoice cryptographically signed by the same government agency also in charge of literally printing money (or an officially licensed company) and then ideally send it using the now mostly defunct monstrosity that is De-Mail because it made guarantees about end-to-end encrpytion and sender/recipient authentication. Luckily this is now largely irrelevant and most companies just send regular PDFs via e-mail and/or make them available for download.