The releae clause is how much money it takes for the club to stand back & allow the player to sign for someone else, if he wishes. Some clubs set it so high that they expect nobody will pay it. If someone does however, they are powerless to stop it.
If Cavani's release clause is £50 mil, & we offer £50 mil, he is ours if he agrees to sign & Napoli can do nothing about it apart from beg him to stay & hope he turns us down.
I don't think we will pay £50 mil, so unless they are willing to let him go for less in the summer, then I can't see us signing him. But you never know.
Barca recently accepted a bid for Messi.
He chose to stay at Barca.