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.
Manchester United and PSG are more interested in signing Osimhen from Napoli, with PSG seemingly much more in need of a striker to complement Mbappe's transfer to either Real Madrid or unlikely Arsenal as rumours would have it.
Arsenal in the meantime will be keen on signing Barcelona's duo Raphinha and Ferran Torres. Barca are in need of financial support and wouldn't hesitate to sell off these and some of its players to aid its financial problems.