If you want to know why Dana White did the media rounds this week saying Mayweather had to call him if he wanted to fight McGregor, it is because of this standard clause in every UFC’s fighter contract.
During the Term, ZUFFA shall have the exclusive right to promote all of Fighter’s bouts and Fighter shall not participate in or render his services as a professional fighter or in any other capacity to any other mixed martial art, martial art, boxing, professional wrestling, or any other fighting competition or exhibition, except as otherwise expressly permitted by this Agreement.
The big difference between boxers and UFC fighters is that boxers are independent contractors. The promoters work for them, not the other way around. Boxers can do whatever they want when they are not boxing and have the ability to turn down fights if they so please.
UFC is run more like the WWE. Even though their fighters are also independent contractors, they have non-compete clauses in their contracts which prevent them from essentially doing anything unless they have UFC permission.
So, if McGregor truly wanted to fight Mayweather, it would have to be done with UFC’s blessing.
For the record, I don’t think this is anything but Mayweather and McGregor just giving each other free promotion and being attention whores, but if you really think they wanted to fight, it could be done.
McGregor could put pressure on UFC to essentially work with Mayweather promotion to stage a PPV event. Mayweather wouldn’t care as long as he was getting his money and UFC would be dumb to turn down something that would be their biggest PPV to date.
McGregor would probably be the person who suffered the most because a lot of the pie of the revenue would go to UFC instead of in his pocket, which is the reason it likely won’t happen.
McGregor could also go to court and say the contract is invalid because he is an independent contractor, but that likely wouldn’t make it far either, since he voluntarily signed the deal.