An entryman making an exchange under the provisions of this subchapter may be given credit by the Secretary upon any land development charges made by the United States in connection with the lieu farm unit for any such charges paid to the United States in connection with the original unit. A resident owner making an exchange under the provisions of this subchapter may, to the extent, to which he or, in the case of a widow, widower, heir, or devisee, his spouse or ancestor, as the case may be, has paid to the United States the purchase price of the original farm unit, be given credit by the Secretary upon the purchase price of his lieu farm unit; such credit may also be applied in the manner and circumstances provided in
section 451b of this title upon irrigation construction charges for or properly assignable to his lieu farm unit.