if it was a US Postal Service Money Order, then I do believe the Post Office can check to see if it has cleared. call your Local Postmaster and ask.
if it HAS cleared. the US Postal Service takes a VERY dim view of Postal Fraud....
it is possible that the letter from your FFL to the seller got "misplaced" by the P.O. (believe it or not.. it does happen! REALLY! I have friends who work there and tell me it's true! Grin)
your Postmaster can fill you in on the procedure to Cancel the Money Order and just go through the paper work again..
"Question that Regulation, you might be suprised to find out it's Illegal"