Bravo Star Greg Plitt Sued -- I Know You're Dead, But About that $50k ...

Bravo fitness star Greg Plitt allegedly made a handshake deal with a filmmaker to create his popular fitness show, but never paid the guy ... who's now going after Greg's estate to get the cash.

According to docs ... filmmaker Devin Coury says he and Plitt had an oral contract for his online "Raw Series" back in 2009. Coury says he shot the footage for the program, and was owed $50,000 for his services.

Of course, Plitt died last year when he was hit by a train -- but Coury says he never got a dime of the 50 large. In the docs, Coury says he saw Plitt one last time in 2013, and got his word he would make things right ... but that never happened.

As for proof there was really a deal -- Coury's submitted invoices he sent to Greg.

Coury admits trying to get the money from his dead friend's estate is a "fragile situation" -- but he's pressing forward with his claim.