According to ESPN, Mayweather went to the M Resort
The casino tried to figure out if they could legally take the bet while Floyd waited. He then tried to put the money on himself winning by knocking instead, but they didn’t know whether they could take that either. He was getting annoyed at the delay and walked out without placing the bet.
Floyd confirmed that they wouldn’t let him place the bet when talking to ESPN after the fight, but did get some bets in via one of his people: "I think that we bet 100 (thousand) on 9½. 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."
He also said later that he gave a friend US$400,000 to place the bets, but the friend was only allowed to place an US$87,000 dollar wager.
Source: GMB / Bloodyelbow.com