(b) That's probably wrong. In this case, although the breach has not already happened, B has a reasonable suspicion that it is about to, and if it did, he would not be able to recover the very substantial losses that would follow. In these circumstances, a court is likely to issue an order, called an injunction, to stop the threatened breach of contract by A. The order would forbid A from doing what he promised not to do. Such orders are relatively easy to enforce because it is easy to establish whether the court's order has been contravened.
Note that, in this case, the injunction sought does not affect A's other promise to B, that is, A's promise to play football for B's club.