im charged for rcs's mistake

A payment from a customer was loaded into my account (A fault from RCS). a couple of months later is was contacted by the rcs group and they requested to reverse the payment as it was a mistake from their side. i then said it is fine and as long as i don't owe any money, as i wasnt before the mistake, it is then fine with me. i did not spend any of the money that was incorrectly sent to me.

Now i received a statement With a balance due.

POOR SERVICE!!!!!!!!!