...
I mean Bayern could have a big part of it without a doubt, they could offer him amounts that he can't refuse, more than any other team in Europe (they probably can't offer more than some Saudi club) so it is still having to do with them a lot. If Real offers him 40 million a year for example, not that Real Madrid needs Musiala, I am just giving an example, and Bayern Munich offers him "only" 20 million, then yeah he could leave. So Bayern Munich has a responsibility to keep offering him the best contract they can afford to offer, without that they are not going to get much.
There could be some other teams who will chase after him, but as long as Bayern offers a good amount, I do not see him doing anything that crazy, feels like it would be just fine. I understand that it is not going to be just that easy to handle, but it's doable. Bayern Munich makes so much money, paying a part of that to Musiala is something they can afford if they want to keep him.
Of course, but affordability is not only a question of money, it is also a question of consistency within the whole team. If they undermine a reasonable salary structure only to keep Musiala in the team, you can imagine what other players will ask for when their contract is about to expire. Paying Musiala 40 million while 85% of the players get less than half of that is probably leading to a whole new set of problems. Real Madrid and Manchester City as far as I know are playing in a different league already in regards to salaries.
I still think that Bayern Munich is of course a great team, but when Real Madrid or Manchester City is knocking on your door and your next coach could be Ancelotti or Pep Guardiola, it's most likely not very easy to refuse those offers.