When a club demand an unreasonable price, it's mean the club isn't want to let their player to leave, but if the club can fulfill the price, AC Milan don't mind about it because they can sign 2 good strikers after the trade completed. Leao is just a potential good young player, it's still potential which mean he could become as good as Osimhen, but I don't expect he will reach Mbappe or Haaland's level. Or Leao will become a bad player like Nunez in Liverpool where he was a very good player in Benfica.
He became an important player in Ac Milan body in the last 2 seasons so it's no wonder ac milan set a high release clause for him. Leao will stay with AC Milan until 2028 when he signed a new contract this week and the speculation over his future has ended. He got what he wanted at Ac Milan and i think his future will be pretty good at ac milan in the season to come.
Apart from that there were several clubs interested in him but leao only wanted to stay with Ac Milan which made other clubs have to bite their fingers. In fact i think Ac Milan will move forward to get another sticker next season to find a proper duel for Leao.
I think it is not up to Milan to make that decision, if the player wants to leave he could insist on being sold to a bigger club and the team has to accept.
If not, and if the team forcefully keeps the player, two things will happen, first of all the player will say he is "injured" a lot more frequently, he could just say his back hurts and you can never find it, there is no medical technology that can deny it, it is just his back hurting, and you have to sit him out of those games, it can be something else I just gave back as an example, secondly the moment his contract is over he will leave for free and you will never earn a dime off him. So, it is better to sell when a player wants to get out, so it is all on Leao to decide if he wants to leave or not.