Interest Invoice in Payables

Suppose if we are not using interest invoice functionality in Payables but client do receive some interest invoices from supplier,now these invoices would be coming through scanning and then get merge with other invoces.Is there any way or report through which we can figure out at month end or later than this much amount paid on those interest invoices.

How system will identified these interest invoices now ,what can we do different?

Regards,

Neha