I think the most important part of the question is - do you have a responsible person doing this or does anyone just post willy-nilly?
Debtors payments cannot be reversed in the POS as it effect your business on that day. I suggest you create an "Unallocated Debtors" account & post your payments there if you are unsure!
Then in the 4POS BackOffice you can simply do a Debit Journal out of the "Unallocated Debtor" account to the correct debtor account.