This is one of those instances where an analogy can be instructive. Imagine if someone trying to buy Liverpool Football Club gave a ‘cast-iron’ guarantee that they wouldn’t sell Fernando Torres but then the existing owners did before selling up. We wouldn’t then think the new owner had broken their guarantee if they didn’t re-sign Torres.
Having said that, Cameron must honour his pledge that "No [EU] treaty should be ratified without consulting the British people in a referendum." Any further treaties must be put to a vote, a move that would not only give the people the say they deserve but also strengthen the hand of Britain's negotiators at European summits.