Level 1

A/R shows old balance but customer has zero balance and invoice is marked as paid

I have a customer whose last invoice was in a previous year and was paid with credits accumulated from years prior to that (they always overpaid a little each month).  The invoice is marked paid and all credits have been applied.  The customer balance is zero.  For some reason the A/R shows the amount of this invoice still years later.  How do I clear this up without messing up prior year financials?  We are on cash basis but use invoices to keep track of our customers.