Sancho's rumoured 25 million euros fee isn't that huge honestly. The real problem is his salary... Why?
Because he makes 7.8 million pounds (~9.2 million euros) a year at Manchester United. Chelsea didn't even pay his full salary during their loan contract.
Is Napoli or Juventus keen on paying this much money per year to him?

I don't know whether he is keen on taking a pay cut.
Napoli can afford to pay Sancho's salary, but i don't think that's good for their team. There will be an internal conflict if they pay someone who has been so mid to receive the highest salary in the squad.
I think they must think for twice before they give him such huge wages. That's obvious he won't also agree to receive paycut.
He lft from Chelsea due to the paycut. It's similar problem to the his transfer to the Napoli too.
If Sancho refuses to take a pay cut at Napoli, I think Napoli will not pay Sancho's asking salary because Sancho has one year left on his contract with Manchester United. Napoli will prefer to wait for Sancho's contract with Manchester United to expire so that they can sign him for free and negotiate for a new salary that will be easy for them to pay Sancho.