As far as I understand, Bonato, sporting director of Sassuolo, explained the situation back in February. We essentially own only right of first refusal, and we can buy 100% of him for 15m during first 10 days of June, or 20m afterwards. If we choose not to buy, effectively he's wholly owned by Sassuolo, the way his ownership can pass to them entirely is complicated, there is also the possibility of a blind auction involved somehow which his agent mentioned.
To avoid all that we should exercise our right and buy the entirety of his contract, so we can then loan him back or whatever else.