The payment of all construction charges against said areas temporarily unproductive shall remain suspended until the Secretary of the Interior shall declare them to be possessed of sufficient productive power properly to be placed in a paying class, whereupon payment of construction charges against such areas shall be resumed or shall begin, as the case may be. Any payments made on such areas shall be credited to the unpaid balance of the construction charge on the productive area of each unit. Such credit shall be applied on and after April 23, 1930, which shall not be construed to require revision of accounts adjusted before April 23, 1930, under the provisions of this section as originally enacted. While said lands so classified as temporarily unproductive and the construction charges against them are suspended, water for irrigation purposes may be furnished upon payment of the usual operation and maintenance charges, or such other charges as may be fixed by the Secretary of the Interior the advance payment of which may be required, in the discretion of the said Secretary. Should said lands temporarily classed as unproductive, or any of them, in the future be found by the Secretary of the Interior to be permanently unproductive, the charges against them shall be charged off as a permanent loss to the reclamation fund and they shall thereupon be treated in the same manner as other permanently unproductive lands as provided in sections 423 to 423g and 610 of this title except that no refund shall be made of the construction charges paid on such unproductive areas and applied as a credit on productive areas as herein authorized.