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.