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.
Manchester united are the ones overpaying these players for nothing, and they have become a pain on the ass for United in offloading them. Sancho should accept a pay cut and leave United since he's not adding any value to the club. Maybe, when his contract expires, he will go for a higher pay which I don't think is possible. No club will pay him what United is paying him with his current performance.
No team will be willing to pay a player whose quality has declined but I am sure Sancho will be willing to lower his salary and this is for the sake of his career because he is still very young and there will be many possibilities in the future for his career.
Manchester United have indeed made many mistakes but now they can gradually correct their mistakes even though they have to bear many losses but I think it must be done for the stability of the team and it is appropriate for Manchester United to sell players who are not included in the coach's scheme to build the team but I regret it a little because Sancho is a good player.