Any entry or purchase contract land with respect to which a loan is made under the authority of this section and section 1006a of this title shall be subject to cancellation by the Secretary of the Interior as provided by existing law or upon request of the Secretary of Agriculture whenever default occurs in the terms, conditions, covenants, or obligations contained in the mortgage. After cancellation or relinquishment of an entry or purchase contract, land on which there is a mortgage lien, pursuant to the provisions of said sections, shall thereafter, except as hereinafter provided, only be open to entry or resale to persons eligible for bo