Promissory estoppel
Contract Law

Promissory estoppel is an equitable doctrine that arises in the context of an existing contract: a promise, assurance or representation of some future conduct by a person is relied upon by another, such that a person relies on the promise of conduct to their detriment, and the promisor will not strictly rely on their strict legal rights, or a defence that would otherwise be available. The promise, assurance or representation must be clear and unequivocal. In essence, estoppel is a rule of evidence that prevents a party to legal proceedings denying the truth of a statement made by them.

An objective test is applied to determine whether it would have been reasonable to rely on on the representation, and mere threats are inadequate.  The promise itself does not need to be the sole inducement but it must form some part of the inducement upon which the defendant has relied.  The requisite reliance will often be made out by expenditure of some form. If there is no reliance by the defendant, then there is a good argument that it is not inequitable for the claimant to go back on their word.