Floyd Mayweather says he attempted to place a $400k bet on himself, but the casino wouldn't allow him to place the wager. (0:34)

Not everything went right for undefeated boxing champion Floyd Mayweather on Saturday night.

Multiple sources told ESPN that Mayweather went into the sportsbook at the M Resort in Las Vegas at around 3:30 p.m. PT Saturday with cash and plans to bet on his fight against UFC star Conor McGregor.

Mayweather attempted to place a $400,000 bet on the fight ending under 9.5 rounds at -200 odds, but according to a source, there were concerns over the legality of a fighter betting on anything other than a straight win, which caused a delay.

Mayweather then tried to bet on himself to win by knockout, but was unhappy about having to wait to place the wager and left without getting the bet down, the sources said.

"He was pissed," one source said.

The sources weren't sure whether he placed the bet at another sportsbook.

Mayweather, who went off as a -500 favorite at most sportsbooks, stopped McGregor 1 minute, 5 seconds into the 10th round. Mayweather would have won either attempted bet: under 9.5 or to win by knockout.

"I think that we bet 100 on 9½," Mayweather told ESPN's Sal Paolantonio after the fight. "I gave my guy six figures to go bet. I have to make sure he bet because earlier today I went to the sports book to bet and they wouldn't let me bet."

Mayweather later told SportsCenter he gave a friend $400,000 to bet, but that the friend was only allowed to place a bet for $87,000.

Neither the Nevada State Athletic Commission nor Nevada Gaming Control has regulations addressing a fighter's ability to bet on himself.

CG Technology, which operates the sportsbook at the M Resort, said it would not comment on an individual customer.