Manny Pacquiao would have already grab that opportunity if he really think this fight is good for his legacy.
I know it's also about the Money but I never heard Pacman mentioned of fighting a UFC fighter just to make more money even how popular they are.
If Manny Pacquiao is looking to set his legacy then he needs to have the rematch between Mayweather and that too outside Nevada state commission, the only UFC fighter that is capable of pulling huge crowds is Conor McGregor and i am sure Manny will school him and finish him in the first 5 rounds and it will be a good easy pay day for Manny and why would anyone turn down a money fight.
5 round? I doubt that Manny can do that, Floyd was extended to 10 rounds by Conor. I think one reason is that Floyd wanted to entertain the crowd. But I don't think that Manny is interested on that money fight though, Floyd or any top welterweight next year will be the goal.