I think it makes no sense for a senator to keep fighting, I don't know what type of country would just watch a senator fight for money but that is the case right here. Even if this happens and then Mayweather wins again which I am sure he will, what is it other than money they could potentially get from this, they are already quite rich and the money Pacman makes from this fight translate to his country means richer than Mayweather makes and takes to USA since life is more expensive there so money should not be a problem.
Even if it is, they could potentially just do "winner gets 300, loser gets 100" type of so that both will work on winning the match. Aside from that there is really no point for this fight and I would say it should not happen. They are well beyond their retiring ages anyway.
Mayweather has been out of the ring for several years now (competitively), so it's not so sure he'll beat a rough and ready Pacquiao.
Plus Mayweather is a few years his senior, and we all know that athletes start deteriorating in their late 30's, so Mayweather may be far below his prime.
Most of the time the fighters get a fixed purse, obviously Mayweather would get far more than Pacquiao since he manages himself, plus he would still be the A-side.
Usually the winner would receive a win bonus too, which is often a cut of the PPV earnings and promo revenue.
However, if you had the chance to earn $25m+ in a fight, you would do it too. Fighting sure as f*** pays more than being a senator!