I may be missing something, but I haven't seen an actual FDCPA violation here. Suing for less than the amount owed isn't an FDCPA violation, as far as I can tell.
That being said, if you offered a mutual dismissal, and they countered with you paying them, you appear to be at an impass.
The most likely things that can happen are:
1. They never pay the fees, the case is dismissed from arbitration. You win.
2. They pay the arbitration fees, and continue with the case. Before the hearing is scheduled, they will get a bill. You make your offer to them again. They accept,