Griezmann has informed Atlético Madrid that he will not be attending pre-season training as he is going to pay his €120m release clause this week. It is possible, like Neymar did, it’s the player who terminates his own contract. It’s obviously not their money, but they’re the ones there with lawyers paying it. When a player buys out his own contract, he's a free man. Likewise, Griezmann can join wherever he wants & legally Atleti can't do anything about it, even if he joins Barça.