Where a power to revoke a conveyance of any lands or the rents and profits thereof and to reconvey the same shall be given to any person other than the grantor in such conveyance, and such person shall thereafter convey the same lands, 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 were recited therein and the intent to revoke the former conveyance expressly declared.