In the absence of a contrary requirement in the contracts between the United States and the water users organization or district assuming liability for the payment of project construction charges, all sums collected under sections 424 to 424e this title from the sale of lands, from the payment of project construction charges on “temporarily unproductive” or “permanently unproductive” lands so sold, and (except as stated in this section) from water rentals, shall inure to the Reclamation Fund as a credit to the construction charge payable on May 16, 1930, by the water users under their present contracts, to the extent of the additional expense, if any, incurred by such water users in furnishing water to the unproductive area, while still in that status, as approved by the Commissioner of Reclamation and the balance as a credit to the sums heretofore written off in accordance with sections 423 to 423g and 610 of this title. Where water rental collections under sections 424 to 424e of this title are in excess of the current operation and maintenance charges, the excess as determined by the Secretary, shall, in the absence of such contrary contract provision, inure to the Reclamation Fund as above provided, but in all other cases the water rentals collected under sections 424 to 424e of this title shall be turned over to or retained by the operating district or association, where the project or part of the project from which the water rentals were collected is being operated and maintained by an irrigation district or water users association under contract with the United States.