The 22-year-old Alvarez, the 2010 ESPN.com prospect of the year, is Mexico's most popular fighter and is coming off his biggest win. He claimed a unanimous decision against Austin Trout on April 27 to unify two 154-pound world titles before a crowd of about 40,000 at the sold-out Alamodome in San Antonio.

"I'm very excited. This is the fight I have wanted for a while," Alvarez told ESPN.com from the Golden Boy Promotions offices in Los Angeles after signing his contract. "It's been talked about for a while now, and now that it's signed, sealed and delivered, I am very happy.

"I will be the first to beat Floyd."

Coming to an agreement on the specific weight was a major issue in making the deal. Mayweather often mocked Pacquiao for insisting on catchweights for some of his fights, and in the two previous junior middleweight fights Mayweather has had, against Cotto and Oscar De La Hoya, the contract weight was 154. But this time, Mayweather sought the catchweight.

"It's 152, and that is fine," Alvarez said. "I will have to work a little harder, but I feel I can make the weight and that I will be strong. To make the fight I had to give a little, he had to give a little, too. I feel it was fair, and both sides are happy. He fought [with a contract weight of] 154 before, but he wanted the [catchweight] and I accepted it. He gave a little, I gave a little, and we got the fight done."

Alvarez, who won a vacant world title by easily outpointing Matthew Hatton in March 2011, has made six defenses, including the unification win against Trout. No. 7 against Mayweather, a supremely conditioned fighter with defense that ranks with the best who ever have boxed, figures to be his most difficult opponent so far, but Alvarez said he's ready.

"We all know and realize that he is the best," Alvarez said. "But I'm gonna prepare myself like never before. I think that I'll have my own speed and intelligence and strength behind me to figure him out and be able to win."

With the fight taking place on Mexican Independence Day weekend, a traditional date for a major fight involving a top Mexican boxer, Alvarez should be the crowd favorite even though Mayweather lives in Las Vegas and will be fighting at the MGM Grand for the eighth consecutive time (and 10th straight in Las Vegas).

"It's a great satisfaction and a great honor to be fighting on this date and to have the Mexicans behind me on such an important day," Alvarez said. "Losing is not an option to me. I am expecting to win and then to be celebrating afterward with all of my Mexican people."

The bout will be the second of the 30-month deal for as many as six fights that Mayweather signed earlier this year with Showtime/CBS, and it figures to be a slam dunk to generate 1 million pay-per-view purchases.

Showtime announced the fight with Guerrero did "in excess" of 1 million buys, but no figure was released and multiple industry sources said it really did around 870,000 to 900,000 buys.

Five of Mayweather's previous six fights topped 1 million buys, including his 2007 fight with De La Hoya, which set the all-time record with nearly 2.5 million.

Perhaps the relatively weak performance of the Guerrero fight convinced Mayweather to pursue the bout that would generate by far his biggest buy rate.

Guerrero was relatively unknown and did not have Alvarez's fan base. That fight also badly stumbled out of the promotional gate because Mayweather was finalizing the deal with Showtime/CBS -- estimated to be worth perhaps as much as $200 million -- and was announced so late that there was no kickoff news conference or media tour to promote it.

The Mayweather-Alvarez fight, however, promises to be much bigger.

Floyd Mayweather Jr. is giving fans what they want: a matchup with unbeaten Canelo Alvarez, right, on Sept. 14. Getty Images

"This is a gigantic fight, and it will be a tremendous promotion," Ellerbe said. "You have 'Money' Mayweather, who is the best in the sport, and you have Canelo, the guy everybody wants to see attempt to knock Floyd off his throne. It gets no bigger and better than this."

Mayweather and Alvarez have been on a collision course. Alvarez was supposed to fight Trout on Mayweather's May 4 undercard with the stipulation that as long as they both won they would face each other in the fall.

Mayweather, however, declined to finalize the agreement, leading Alvarez to face Trout on his own card on April 27.

Schaefer said getting the September fight finalized was difficult.

"I kept working and working," he said. "On Monday, when everyone was at their Memorial Day barbeque, I was in the office working on both sides. Then I felt I got it to a point where I could get Canelo to sign, but when they came in [on Wednesday], they were not OK with certain aspects of the deal."

At that point, Schaefer said it was time for a decision because he is flying to New York on Thursday to attend the wedding of former junior welterweight titleholder Amir Khan this weekend.