Apply Receipts to Interest (not calculated)

If you create a receipt for $500, and apply to an invoice for $498, how can you classify the unapplied $2 as interest?  We will not be calculating interest on our invoices, but may receive interest from some customers.  Any overpayment we would like to identify as interest.  I tried to create a receivable activity but I dont' see it in the Apply To drop down box on the receipts screen.  Our On Account is setup to a clearing account so I don't want to have any interest go to clearing if possible.

Thanks!