Jules Wainstein's ex tried to pull a fast one by renting out an NYC apartment he had no business hawking because SHE owns it ... according to a new lawsuit, but he denies any wrongdoing.
Michael Wainstein is being sued by Jonathan Sheffer -- a famous conductor -- who says he needed to lease a place for his nanny, and Michael offered the pad ... at a premium. In the suit, Sheffer says Michael demanded more than $20k for rent, security deposit and broker fees.
Sheffer agreed and signed the lease, but according to the docs ... he had maintenance issues with the place, which led to his big discovery -- Michael wasn't the owner.
Sheffer thinks Michael was taking the cash for personal use. He's suing for a complete refund, plus damages.
Wainstein's attorney tells us Sheffer's the one in the wrong, and claims he breached the terms of the lease and is simply looking for "a creative way to cancel his obligations." He adds ... Michael denies all the allegations, and plans to file his own lawsuit against Sheffer.