You know a lot of clubs are actually ready to spend a lot of money for him. So it might be a problem for them.
But to mean the bigger question is that, is he actually willing to go to Chelsea or Manchester United?
Chelsea or Manchester United? my answer is PSG.

Even both Chelsea and Manchester United afford to buy out Osimhen, but they need to pay his high salary too, so this is why I say PSG is more realistic. PSG can buy out him and able to pay high salary.
It just depend on Osimhen decision, will he want to play for career or play for money.