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.
I think that Juventus should pay the money and bring Sancho to turin because Juventus really needs to strengthen their team because they have not been performing that great due to lack of quality players in the team because 25 million is not too big for a player like Sancho