"I have instructed my promoter, Bob Arum, head of Top Rank Inc., to help me out in the filing of the case as soon as possible because I have had people coming over to me now asking if I really take performance-enhancing drugs and I have cheated my way into becoming the No. 1 boxer in the world,"

Pacquiao to sue Mayweathers in doping row | Reuters

If the fight doesn't happen, it's pretty damning for the Mayweather camp especially Mayweather Sr. At minimum he'd be looking at defamation and maybe even a libel suit for what $20 Million? Now that's no joke!

Defamation - Wikipedia, the free encyclopedia

Even if the fight does happen, Pacquiao might still have a rock solid case.