Need to reverse document that was done through IDOC. Cannot be reversed in Financial Accounting?

We use a custom program/report to upload payroll information.

We need to reverse one of these documents.

When using FB08 to do the reversal, we are being told that the document cannot be reversed in financial accounting. The details say that it was an IDOC.

Any hints on getting this reversed? Does this mean I need to get another custom program to do the reversal?