I think the big beef with PSG last year was the idea that Neymar was paying his own buy-out clause, which he covered by getting paid the same amount to sign a sponsorship deal with Qatar for the WC 2022. I don't know the exact details of how it was worked out, but it seemed to be a pretty blatant flaunting of the rules.
Similar thing happened with Vitolo and Atletico, just on a smaller scale. Player received 40m EUR, paid his own release clause and went to Atletico (well... sort of... Atleti was banned from incoming transfer until the winter, so first he went to Las Palmas for a few months).