Rihanna & Jay Z Sued: She's Unfaithful ... to Fans and My Pockets

3:35 PM PT -- A rep for Roc Nation tells TMZ ... "Rihanna, Roc Nation nor anyone associated personally or professionally with either party was in contact with this person. Unfortunately this person was scammed. Rihanna nor Roc Nation collected any money for this event."

Rihanna flaked on a show she was scheduled to perform at in Nigeria, even though she'd already received a fat chunk of her payday ... according to the guy who booked her for the gig -- but Rihanna is claiming BS.

Chris Ubosi and his company, Megalectrics, are suing RiRi over a 2013 concert that never happened. Ubosi says he struck a deal with 2 people who claimed to rep Rihanna, as well as her label Roc Nation and Jay Z. Rihanna would get $425,000 to play for 65 minutes.

The show was supposed to go down in May 2013 -- and Ubosi says he made 3 payments, totaling $160k ... but then her camp asked to postpone the show. Ubosi says he agreed, but only IF Rihanna posted the new date on social media.

In the suit, he says that never happened so Megalectrics pulled the plug, and demanded its money back. Since Ubosi didn't get the cash, he's suing Rihanna, Jay Z and Roc Nation for the dough.