From what I've read around, it seems that the deal fell through due to the medical check-up carried out by Juventus. It seems that the player didn't pass the medical tests, but maybe I'm wrong. I can't find the article but it seems to have said exactly this. I wouldn't be surprised if they would have increased the checks after the previous purchases.
I thought Juventus dropped out of the race to sign Sancho because of the player they signed, and they didn't have enough money to pay for Sancho's transfer fee at the moment. I have been following transfer news for a few days now, and I haven't heard that Manchester United and Juventus have agreed on a transfer fee for Sancho. How sure are you with this information?
I think it is worth noting that earlier reports suggested Juventus had reached an agreement with Manchester United for Sancho's loan with an obligation to buy, well i think these claims seems to have been superseded by the news of Juventus withdrawals, too me i have a fewling that Juventus seems to have prioritize development of thier young talent, particularly samuel mabangula, over signing Sancho, Mbangula's rapid growth and impressive performance likely influenced Juventus decision,Manchester United insistence on covering a significant portion of Sancho's wages as part of any loan deal might have played a role in Juventus withdrawal.