Regarding open items in credit management

Dear friends

My query is that is it possible to restrict credit management for credit and debit open items? See i am using oldest open item blocking in my scenario. Now credit and debit amounts are both considered as open item for credit management. I want only invoice (Debit entry) for consideration and not his payment(credit entry).

Please advise is it possible or not and if possible where to define that???

With regards

Inder