The Secretary is authorized and directed to investigate the repayment problems of any existing project contract unit in connection with which, in his judgment, a contract under section 485b or 485c  of this title would not be practicable nor provide an economically sound adjustment, and to negotiate a contract which, in his judgment, both would provide fair and equitable treatment of the repayment problems involved and would be in keeping with the general purpose of this subchapter.
For any project, division of a project, development unit of a project, or supplemental works on a project, under construction on August 4, 1939, or for which appropriations had been made, and in connection with which a repayment contract had not been executed, allocations of costs may be made in accordance with the provisions of section 485h of this title and a repayment contract may be negotiated, in the discretion of the Secretary, (1) pursuant to the authority of subsection (a) of this section or (2) in accordance, as near as may be, with the provisions in section 485h(d) or 485h(e) of this title. In connection with any such project, division, or development unit, on which the majority of the lands involved are public lands of the United States, the Secretary, prior to entering into a repayment contract, may fix a development period for each irrigation block, if any, of not to exceed ten years from and including the first year in which water is delivered for the lands in said block: Provided, That in the event a development period is fixed prior to execution of a repayment contract, execution thereof shall be a condition precedent to delivery of water after the close of the development period. During any such development period water shall be delivered to the lands in the irrigation block involved only on a toll-charge basis, at a charge per annum per acre-foot to be fixed by the Secretary each year and to be collected in advance of delivery of water. Pending negotiation and execution of a repayment contract for any other such project, division, or development unit, water may be delivered for a period of not more than five years from August 4, 1939, on the same toll-charge basis. Any such toll charges collected and which the Secretary determines to be in excess of the cost of operation and maintenance during the toll-charge period shall be credited to the construction cost of the project in the manner determined by the Secretary.
The Secretary from time to time shall report to the Congress on any proposed contracts negotiated pursuant to the authority of subsection (a) or (b)(1) of this section, and he may execute any such contract on behalf of the United States only after approval thereof has been given by Act of Congress. Contracts, so approved, however, may be amended from time to time by mutual agreement and without further approval by Congress if such amendments are within the scope of authority granted prior to or after April 24, 1945, to the Secretary under any Act, except that amendments providing for repayment of construction charges in a period of years longer than authorized by this subchapter, as it may be amended, shall be effective only when approved by Congress.