How sponsor bugs?

I reported several bugs in Mantis and offered a small sponsoring for some of them. The first one is now marked as “resolved”. How do I pay the money now? Just a donation with the promised amount?
But in paypal I see no possibility to leave a comment regarding the bug number. So there would be no “control”, if I have payed for this bug because my paypal email is different from the email I used for registering in Mantis and the Ardour forum.