Mayweather said himself that if Shane wanted the re-match then he would give it to him.
It makes no sense for Mayweather to fight Mosley against after he put a boxing clinic on May 1st. Mayweather can fight someone and make more money. If they have a re-match, the fight will probably sell under a million.
I doubt shane would want that to be honest.
I dont think floyd will fight at welter again if he cant get pacquiao. He'll move up to 54 and outbox a paper champion (possibly the winner of cotto/foreman) or he'll go to 160 and box Martinez.