The construction charges prior to May 25, 1926, paid on permanently unproductive lands excluded from the project shall be applied as a credit on charges due or to become due on any remaining irrigable land covered by the same water-right contract or land taken in exchange as provided in section 423c of this title. If the charges so paid exceed the amount of all water-right charges due and unpaid, plus the construction charges not yet due, the balance shall be paid in cash to the holder of the water-right contract covering the land so excluded or to the irrigation district affected; which in turn shall be charged with the responsibility of making suitable adjustment with the landowners involved. Should all the irrigable lands of a water-right applicant be excluded from the project as permanently unproductive, and no exchange be made as provided in said section, the total construction charges paid before May 25, 1926, less any accrued charges on account of operation and maintenance, shall be refunded in cash, the water-right contract shall be canceled, and all liens on account of water-right charges shall be released.