Sales invoice in foreign currency

Invoice in euros, I would think? You don't have to do anything special, Belgian accounting, so just invoice in euros.

Hello,
Thank you for your answer. As I said, I invoice that customer in British pounds. It's been that way for over ten years and that's what we agreed.
I think I'll create the invoice in Dexxter (i.e. in EUR) and state the total amount in British pounds from all invoice items in the notes on the invoice (based on the exchange rate on the invoice date).

If you want to make it easy for the customer (or if it has been agreed), you can add it yourself. The customer is king, after all :)

Haha, yes, it is indeed a case of "the customer is king". Thank you!