A term in a contract is only enforceable if you're prepared to go to court to enforce it. By which time the transfer window would have been closed. (I think it would have actually gone to the FA and then FIFA and then the CAS).
But - and I can't find a reference now - but I know I read it at the time, under English law, release clauses are not considered legally enforceable. The argument goes something like this:
The contract is between the player and the club. Therefore it is not binding on the club in negotiations with, say, Arsenal since they are not party to the contract.
They are enforceable however in cases where potential buyers are named within the contract - which apparently makes them parties to the contract. This is what happened with Joe Allen whose contract named a price and each of the top six clubs in the Premier League.
And they would be enforceable - as in Spain - where they are "buy out" clauses between club and player. See the Herrera transfer for more information on that. I think he had to receive the money from United and pay it to his club?