Where a power to revoke a conveyance of land, or the rents and profits from it, and to reconvey the land or the rents and profits is given to a person other than the grantor in the conveyance, and the person subsequently conveys the land, rents, or profits to a purchaser for a valuable consideration, the subsequent conveyance is valid in the same manner and to the same extent as if the power of revocation were recited in it and the intent to revoke the former conveyance expressly declared.