negative invoice value / repayment

[tereque]

hi,

I have issued an invoice to a customer who has paid the full amount. It 
has turned out that a part of this order can not be delivered, so I have 
to return a certain amount of this payment back and issue a new invoice 
which in the end shows a negative amount.

problem 1: gnucash does not allow a negative invoice amount
problem2: how do I have to do a payback

I have posted an invoice to a a/r account and paid that invoice using 
the 'pay invoice' function

any advices how to handle such a situation?

many thanks as always ... Bela