On loan, with an option to buy.
Sanches also spent last season on loan at Roma, who chose not to buy him for €15 million after he seemed to have a disappointing time there. He's not a prized asset for PSG, though he may have more value to Benfica given he started his career there.
Anyways, Neves had a release clause of €120 million but Benfica, who are notoriously difficult negotiators, accepted an offer of almost half that. It's just a surprising deal given how highly-rated Neves is and how many clubs across Europe were looking for a midfielder this summer, yet PSG seemingly had a free run at him at a very good price.