Credit note

When I create a credit note, my income on the dashboard decreases. But only my income; my outstanding sales invoices remain the same, as do my outstanding purchase invoices. It's not the case that a credit note immediately reduces the outstanding amount of sales invoices, which I understand. In your case, they cancel each other out, but sometimes that may not be the case.

Then you have to set the sales invoice and credit note to 'paid' status, I think?

Yes, that was indeed the solution... Thank you very much! I hadn't thought of that, but it makes perfect sense!