There is no reason to suppose that the ordinary principles of estoppel do not apply to the formation of contracts. If, therefore, either party misrepresents an element of fact essential for the existence or non-existence of a contract, and the other party justifiably relies upon this representation and takes detrimental action in consequence, it will not be open to the party making the representation to show his error. Thus, if an acceptor dates his acceptance at a date beyond the last moment when the offer would be open, and, relying on this date, the offeror changes his position, the acceptor could not show that the date was an error and that the acceptance was in fact seasonably dispatched.49 So, it would seem, if after a letter of acceptance had been mailed completing a contract, the acceptor should telegraph a rejection of the offer, and relying on this rejection the offeror should change his position before the receipt of the acceptance, the acceptor should not be allowed to show that he had in fact accepted the offer while still open. It is essential, however, for the creation of an estoppel that there should be a misrepresentation of existing fact; it is not enough that relying upon a promise of future performance, the promisee changes his position.50

49 But if the offeror takes no action in reliance upon the erroneous date, the accaptor could show the truth. Dunlop v. Higgins, 1 H. L. C. 381; Stockham v. Stockham, 32 Md. 196, 208. 50 See infra, Sec. 690 and as to estoppel to deny consideration, infra, Sec.Sec. 115a, 139.