In my head, he's not exactly essential, so my starting position would be to balance the risk with gradual improvement of terms based on performance.He has 4 years left on his contract at Juve, he'll try to negotiate a long term contract just like every player. If you pay £25M for him (well you dont really pay it but that's the value the club put on Sancho), then at the very least you'll offer a 3 year contract.
I dont know anything about him outside of the pitch and it depends on the personality of the player but you've seen it with your own players, not every player is willing to take a paycut.
If he's not interested, walk away. We don't need him desperately so we don't need to pay him a fortune or gift him a lengthy contract.