I'd wait for further details but it's a lot more complicated as a lot of South American deals are when you're buying directly from the source instead of waiting until their world class.
He's originally from Estudiantes then somehow his rights were bought by a Uruguayan club who then loaned him to Sociedad for a year, then loaned him for another year and was looking to buy him outright following the end of this season. Enter us out of nowhere who had more money to buy him outright but there might be a contract situation between us and Sociedad now that's trying to get worked out.
There's also the situation between Hart/Willy. Hart's days as # 1 could be numbered. If I had a guess he's probably the surprise big name that could be gone but Willy also has a year left. Depending how things go with Joe, he can improve his chances of staying number 1 by having a world class season or there's Rulli who can continue to develop into the next top keeper in the world.(Olympics this summer, another top season at Sociedad) and we can still end up getting him cheaper than looking elsewhere and paying £40-60M on a keeper somewhere else.
Reminds me of how clubs like Barca and Juventus buy players in South America but also get first options on literally all the rest of the top prospects at the club for cheap. There's a mention in this AS article of having an option on 19 year old winger Mikel Oyarzab who's highly rated at Sociedad included in this deal.
Again I'll wait till things are more official but this is what it's looking like to me