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.