PSG has signed him with a lot of money. It's natural to see the club is not even trying to lose it again. Neymar and mbappe have become two players who are breaking world record as players with most expensive value that have ever been recruited in the football history. THat's why PSG is not wanna let mbappe leaving from the club for free.
PSG bought Mbappe from Monaco with big transfer cost and after many years, they paid him a lot of money for his salary too. What they got from Mbappe contribution over those years? Only Ligue 1 titles and nothing more and he even asked more from the club.
I believe PSG does not feel happy with Mbappe and they are trying to convince him to stay because they have yet found good alternative players for Mbappe. The player understands the situation well so he tries to bargain with PSG.
PSG can use money from selling him to buy a better player to replace him like osimhen.
Osimhem is a better player as he does not want to a center of his club like Mbappe and he is ready to be a normal piece in a squad and does everything just to help his team getting as best results as possible. So far it is what Osimhen has shown in his career and he can change in future but I think he has shown his basic characteristics and I believe in Osimhen.