Floyd Mayweather Sued By Vegas Nightclub, Allegedly KO'd His Own Bday Party!

Floyd Mayweather was gonna make $35,000 for 90 minutes of work at a Vegas nightclub ... but blew it off because he didn't feel like leaving Europe ... this according to a new lawsuit.

Mayweather's being sued by the SLS Hotel in Vegas -- which says they had a deal with Mayweather to host his 39th birthday party at their Foxtail Nightclub on February 20.

All Floyd had to do was show up ... host the party and do the red carpet -- and bam, easy $35,000.

The SLS says he even got half the money in advance.

Problem is ... Floyd allegedly bailed at the last minute so he could "stay in Europe for his birthday."

Obviously, the club was pissed ... and demanded the advance back, which Floyd has allegedly refused to fork over.

The SLS is suing for the money back -- plus additional damages.