running total for A/R

[Ariel]

How (or where) does gnucash keep track of the running total for A/R?

I have a situation where a client effectively (not actually) got back part 
of a payment they had sent to me, and I have no idea how to enter it.

Do I enter a negative payment? A fictitious invoice?

I tried directly entering something into the A/R account, but that didn't 
change the running total.

A negative payment would be ideal I think, because I don't want to create 
a fake invoice, but do I remember correctly that gnucash doesn't allow 
this? Perhaps it should?

 	-Ariel