Floyd Mayweather is giving Khabib a simple choice -- we fight in the boxing ring ... or we ain't fightin' at all!
Of course, the two have been goin' back and forth since Khabib beat Conor and called out Floyd ... but Nurmagomedov's team AND Dana White both say it's gotta be MMA.
Now, Floyd's firing back ... telling TMZ Sports HE makes the rules -- 'cause he makes the money and that makes him the guy who calls the shots ... and mixed martial arts isn't happening.
"They said it has to happen in the Octagon. And I said, 'Just tell me where Khabib has made 9 figures before in the Octagon!'"
"If he hasn't made 9 figures, he's not the A-side. So, I'm the A-side."
Remember, Floyd told us he's likely to fight on New Year's Eve in Tokyo ... and if he and Khabib can't strike a deal, that's cool with him.
"It's my way, my rules. I'm Floyd 'Money' Mayweather!"