Question regarding ar_receipt_api_pub.activity_application

Hi All, We have a 3rd party interface to receive receipts data. As part of that we not only create and apply the receipts to open receivables but in case the receipt has unapplied amount within a tolerance, we write it off using ar_receipt_api_pub.activity_application. But we are noticing a discrepancy. In cases where the receipt has been applied to an invoice instalment that already had a partial payment, the writeoff is failing with "The receipt amount cannot be less than the sum of the applied and on-account amounts." The write-offs for receipts applied to fully open instalments are working fine.