REI INTERVENTUS. When a party is imperfectly bound in an obligation, he may in general, annul such imperfect obligation; but when he has permitted the opposite party to act as if his obligation or agreement were complete, such things have intervened as to deprive him of the right to rescind such obligation; these circumstances are the rei interventus. Bell's Com. 328, 329, 5th ed.; Burt. Man. P. R. 128.