According to Chris Mannix the Staff Writer for Sports Illustrated that covers the NBA and boxing there is NOT a rematch clause for Mayweather-Mosley
here are his most recent TWEETS
"It's done: Richard Schaefer just told SI.com that Floyd Mayweather has signed a contract to face Shane Mosley on May 1st in Las Vegas"
"Contrary to reports, there is no automatic rematch clause for either fighter in the contract."
"Rising Mexican star Saul Alvarez will be on the undercard. GBP President Oscar De La Hoya will conduct a separate press tour w/Alvarez."