Personally, i don't think that this fight will happen because Mayweather doesn't want to tarnish that zero lose that he had in his career. Though this is only an exhibition game but there will be a lot of pride at stake once the contract will be signed because for Pacman, he want to have his revenge and he want it even in an exhibition match and for Money, he can't afford to lose so that's the reason why this fight will not come into fruition as Mayweather is very beatable by Manny on his current age.
That's the only problem there since Mayweather is always protecting that zero lose record and Its like he is the one who will have a problem to make this match not to happen. Pacquaio is so prepared and I guess the only one he wait is a contract to make this exhibition fight to happen, but since there are rumors release regarding on this possible match up maybe there's ongoing negotiation happen so maybe its good to watch for more updates between there potential match.
Who knows maybe money can work to make this fight to happen and if Mayweather could possibly receive a lot of money on Pacquio's fight maybe this can change his decision.