Promissory Estoppel:

The doctrine of Promissory Estoppel state that whenever an unequivocal promise is made with the intention of creating legal relationship or affect a legal relationship to arise in the future (notwithstanding any pre-existing legal or contractual relationship between the parties), knowing or intending that it would be acted on by other party and is in fact acted on (altering the position of other party, not necessarily detrimentally) then promisor will be abstained from going back on the promise if it will be inequitable for him to do so (i.e. if promisor’s going back on the promise will detrimentally affect the promisee).