Stevie J and his old Bad Boy pal, Mase, were supposed to headline a nightclub after-party ... but bailed out ... and the promoter thinks Stevie was part of a plot to rip him off.
Promoter Albere Correa fired off a letter to Stevie, and 3 guys he believes have close ties to Stevie ... demanding they fork over $20k for allegedly scamming him. In the letter, Correa claims he paid through the nose to book Stevie, Mase and Lil' Kim.
Correa says he shelled out around $30k to a guy claiming to be Stevie's manager, but as they got closer to the September gig ... he realized he'd been paying a guy who couldn't deliver. As Correa expected ... Stevie and Mase were no-shows at the party. He says Kim was there, but only because Correa booked her on his own.
He's not letting Stevie J off the hook, and is threatening to sue him, and his alleged cohorts, if they don't pay up the $20k.
We've reached out to Stevie for comment ... so far, no word back.