Hello,

Run a "Transaction List By Date" report

Add Column: OPEN BALANCE

Filter For: Transaction = PAYMENT

That will show all payments in the date range you set and if they have an open balance. 

Anything that has an open balance means all or part of that payment is still sitting in the customers account as a credit and hasn't been applied to an invoice.  Once you have this report, you can export to Excel, resort by the open balance and then delete any records that dont have an open balance.  Or, you can setup a filter or sort list in excel and just tell it to not include anything with open balance is blank.

picture attached