Where can I look to find how a payment posted?
 
We have a customer that pays several customers' accounts.  They will take one customer's credit and offset it with another customer's invoice and issue a check for the net difference.  
 
Thought we had solved the posting problem by going to GL-P and changing the second AR account to the cash account on the credits.  What has happened now is that the AR Aging still shows all the credits but the AR on the Trial Balance is correct.
 
HELP!  Thank you for any suggestions.

Sounds like the GL is OK but the Aging is the problem.  You can use AR-B and enter Beginning Balance type vouchers and credits to move balances between custoemrs without potsing to the GL.