How to attribute a refund to an invoice that had to be modified?

[John Griessen]

I have to modify some invoices when I can't ship what was billed.  Is there any tidy way in
gnucash to attribute a refund to an invoice?  In other words, I have a payment for $65,
and a modified invoice for $54., so I need to refund $11, and I want that to stay
related to the original invoice...  I can't process a payment for a negative amount though,
even though that is just what I want to do... a refund.

John Griessen