AR Debit Note entry in cash receipt entry - Version 10.2.200.5

Debit Note creation during cash posting.  We use National Accounts in Epicor so when a national account has many customer numbers it allows us to post the check with the invoice numbers crossing over account numbers.  However, when we need to create a Debit Note, it will not allow us to create it if the invoice # it is associated with is zero, if the invoice # it is associated with is open but the customer # on the header screen does not hold that invoice #, etc.  We end up manually processing those payments, adjusting payment amounts to accommodate this etc.  How can the deduction end up on the correct customer number under a National Account concept, even if there were more than one deductions?  **Added – it would be helpful since the debit note is all manual entry, to allow us to enter the customer # we want the debit note to go to as Epicor puts it where it does based on the programming but almost always it is on the wrong account.  It would be great to be able to enter the customer # of where we want the debit memo to post.  How is this being handled by others?