Refund of registration fee to private person

Can't you delete the initial sales invoice? Or is that cheating :p

Or add credit note?

Your first suggestion is not an option because you have to be able to account for every income on your professional account. I also considered a credit note, but then you first need to have an initial invoice, which I don't have because it's a payment from a private person...So you entered it via the daily receipts, just delete it there? There is a bin next to each entry, so you just need to indicate why you are deleting it and you're done.