Rapper Fat Trel Busted for Fake Ass $50 Bills ... Blames Dat ATM Dough

Rick Ross' protege, Fat Trel, got busted for passing off counterfeit bills in a casino, but he says the funny money is just what an ATM spit out to him.

According to the police report ... Trel, who's signed to Rick's Maybach Music Group, was at a Maryland casino early Easter Sunday morning when he handed over nine fake $50 bills to a pit boss. Cops say the easy tell was matching serial numbers on 8 of the 9 bills.

Fat Trel insists he's innocent ... telling police he got the money from a 7-Eleven ATM in D.C.

The rapper was arrested and charged with possession of counterfeit currency ... which means he's gonna have the Secret Service crawling up his butt soon too.