I have been wondering that too. I do not understand how you can have the Webster ruling and a release clause. If all players can buy themselves out of their contracts then what is the point of a release clause? Perhaps it is a mechanism that relates football transfer fees to wages. I believe Anders Herrera moved to United by buying out his contract at Athletic Bilbao. No one seems to know where Messi stands with his release clause so I assume that City and Barcelona would have to agree a transferCan Messi not just buy his contract out if it runs year by year?
The thing is that if the embodiment of Barcelona FC is not allowed the freedom to play for Guardiola to whom they also owe a debt, then Barcelona will be morally bankrupt. They would be a football club whom no one could respect or love. They simply have to allow Messi to play his last years for Guardiola. I am sure City can compensate them up to a limited amount. Go too far and the deal becomes impossible.