When a power to revoke a conveyance of any lands, or of the rents and profits thereof, and to reconvey the same, is given to any person other than the grantor in such conveyance, and such person thereafter conveys the same land, rents, or profits to a purchaser for a valuable consideration, such subsequent conveyance shall be valid in the same manner and to the same extent as if the power of revocation was recited therein, and the intent to revoke the former conveyance expressly declared.