Thanks Softrak Tech Support!

We were ordering the Aged Overdue Payables with the 'Aged as of' option. I switched it to the 'Calculate as of' option and the mystery transaction no longer appears on the report.

I will look into your other suggestions about how to deal with this transaction at a later date. I think the only real problem we were having as a result of the transaction was the fact that the Aged Overdue Payables report didn't balance to the GL. Changing the report settings has fixed that so the immediate problem is solved.

Thanks,
Laura