Spanish buy-out clauses are different to the release clauses we're used to and have a legal standing. In Spain, they are mandatory and a player can buy out his contract for the stated fee and his club can do nothing about it. But he has to do it, not the buying club as I understand it, otherwise there are tax issues with VAT being payable. The clubs can come to an agreement on price to mitigate this but the selling club has the right to demand the full fee, plus VAT, if it wants. Messi's clause is around £180m I believe.