Recalled settlement?

I recently received a debt collection letter from First National Collection Bureau (sparks, NV) offering me a settlement on an old credit card debt. I kept the terms of the initial payment but after spending $14.40 to overnight the payment, paying for and making out a money order to them for $555, I received my money order back in the mail 3 days prior to the due date with a letter attached saying the account was recalled and that I should call a number to Assett Acceptance LLC to see where the account was now. How can they offer a settlement which clearly states in the written communication they sent, could only be revoked if the terms were not kept, which terms I did keep. The payment was received by FNCB Inc on 2/13/2007 which delivery confirmation I have from overnighting the payment through usps. I also have the original communication they sent offering the settlemnt, as well as the letter and postmarked envelope dated 2/14/2007 they sent my payment back in which shows again that they received the payment prior to the 2/19/2007 deadline. Is this legal and what course of action should I take to counter this? It seems as though the communication was a cover-up for some other motive. Why would I be re-directed to another company which was not even mentioned or referenced in the original communication? I've paid a settlement before from a collector and had no problem with it. I even called them once I sent the payment to verify the payment had been received.