[GNC] Accounts Receivable: How do I handle a Tip?

[Eric H. Bowen]

I just completed a job for a satisfied customer. Very satisfied; he 
voluntarily paid $50 over and above the invoiced amount and told me to 
keep it as a tip. Now, how do I account for that?

I'm very leery about hand editing anything to do with the AP and AR 
systems; there's a caution against that in the documentation. And the 
invoice is already posted, I'd prefer not to unpost and edit it. When I 
post the payment received, AP shows it as a "pre-payment," but I'm not 
sure of the proper way to redirect that to an income account.

Suggestions?