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.