Entering a credit

[Robert G Palmer Jr]

I think this question may have gotten lost in the middle of the other  
questions I asked.

How do you enter a credit in GnuCash?

If I create an Invoice for a given amount: e.g. $100, and post the  
invoice
Later, for whatever reason, I decide that the customer only owes $50.

How do I enter a $50 credit in such a way that it appears on their  
Customer Report as a credit?

Thanks,
Robert