(b) That's right. The relevant doctrine in these circumstances is 'promissory estoppel'. Promissory estoppel is a variety of estoppel which arises when one contracting party leads the other to believe that a contract will be made at some time in the future, and allows that other party to act on that belief in a way that would cause them harm if the contract were not completed as promised. If, in the circumstances, if it would be against good conscience to deny the existence of the contract, the party responsible for the promise that it would be created is estopped from denying its existence, even though in fact the normal requirements for contract formation are not satisfied. In the present case, B will not be allowed to deny the existence of a contract with A.
The doctrine of estoppel is a reminder that, although contracting parties are entitled to rely on the established rules of law and use these rules to pursue and protect their own interests, this does not excuse conduct that a court considers is contrary to good conscience. Promissory estoppel is a fairly recent development. The decided cases are rife with opposing views as to the essence of the doctrine and its finer points which are beyond the scope of this module. However the following case provides a good illustration of the basic concept.