The undefeated 42-year-old was signing autographs at the Art of Music shop inside the MGM Grand in Vegas over the weekend ... when a fan asked Floyd to sign a ticket from the 2015 Mayweather vs. Pacquiao fight.

Get this ... fans were NOT allowed to carry cell phones while interacting with Floyd, but when Mayweather saw the item, he told the fan to go out and grab his phone so he could record "something special."

When the guy returned, Floyd put on a show -- saying, "So, what I'm about to do right now is sign 'Victim 48,'" Floyd said ... a reference to Manny being the 48th fighter Floyd defeated.

"So, what I'm gonna go for you ... I gotta cross him out!"

That's when Floyd grabbed a pen and scratched a big ol' "X" over Manny's face!

As we previously reported, Floyd is dead serious about a boxing comeback in 2020 -- he wants two more fights -- and one of his main targets is Manny.

Floyd is a master of promotion and knows better than anyone how to drum up interest in a major pop culture event ... just ask Conor McGregor!

Of course, Floyd handily defeated Pacquiao when they first clashed back in 2015 -- but Manny was nursing an injured shoulder at the time.

Since then, Manny's been VERY impressive in the ring ... defeating top fighters like Adrien Broner and Keith Thurman.

Floyd knows a rematch with Pacquiao could generate another 9-figure paycheck ... and this seems like a very calculated first step to get the ball rolling.

BTW, the fan with the 1-of-a-kind signed ticket is trying to cash in ... 'cause he's put it up for auction with eBay.

Stay tuned ...