I highly doubt that United would be too willing to sell Casemiro just yet. It feels like we are going to see United either keep him, or maybe find someone better than him too, that could be another option. They wanted to get De Jong all summer last year, if they can do that this year, it would both help Barcelona possibly keep Gavi and give him a decent salary, but also would mean United can sell Casemiro as well, would help Bayern too. So all in all that would be a good move between all clubs. But if United doesn't get anyone for that position, they are not letting Casemiro go, that's not going to be a possible option, it just doesn't make sense at the moment, should be replaced before he is gone, after he is gone would make them rush into making a bad decision.
That is just a rumour, no way Manchester United are letting Casemiro go, he is one of the reasons the team has kept a lot of clean sheets this season, he has been a very important part of the team, he is certainly going to be part of the team for next season and even if he did move, no way Bayern is paying that amount of money for a 31-year-old with only one and a half years left on his contract. And Gavi isn't getting any professional contracts unless he stays at the Academy for another two years or moves as a free agent in the summer, or has the law changed?