Other sources are saying it’s not an obligation, but an option that has a very low threshold to become an obligation.

[Loic Tanzi - T2] Jadon Sancho (Manchester United) approaches Juventus : The Italians have agreed on the main lines with Manchester United, on a loan with an option to buy. This loan with purchase option will be validated under very low conditions, information that we are able to confirm.

https://www.lequipe.fr/Football/Actualites/Mercato-jadon-sancho-manchester-united-se-rapproche-de-la-juventus/1503998