In  RM2000  go to Reports>Accounting Reports>Open Invoices and choose the “Only Unapplied Credits  Box”

In RMA go to Reports>Accounting Reports>Lookup> Open Invoices and choose the “Only Unapplied Credit “ Box.  For a shorter report choose Open Balance Summary with Single Line Aging”  Make sure you set the invoice date field back to a date in the past.  It defaults to today’s date in RMA.