We have had that happen before as well. Very rare though. If you push it, sometimes hard, you can still get resolution. Ask to speak to the manager, plead your case, etc. Find out what he/she WOULD approve (I'd assume between 15-22% right now) and if they will apply that one settlement payment towards the new agreement.
Unfortunatly, creditors have near god-like powers and theres no real place to complain to (they would just say "You owe them $X anyway)