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.
In my opinion, he should study and see about players who join Paris Saint Germain, indeed if we talk about money, Paris Saint Germain can pay him more than any other club. However, if we talk about career, I would not include Paris Saint Germain as a club that can make a player's career better. Let's see what happened to Mbappe after joining Paris Saint Germain. Indeed, he got many privileges, but he couldn't get anything special in his career, even though players would definitely want that.
Chelsea is not very good at the moment, so it is quite risky for Osimhen, Manchester United is also the same, but Manchester United is slightly better than Chelsea. If there is another club that is better and interested in him, I think he should consider that. The reason is that the situation of a club sometimes also influences a player's performance, unless the player can be at a very good level of stability, that might actually be what can take the club in an even better direction. But I haven't seen that from Osimhen.