(a) The authority granted in section 485b of this title for modification of existing repayment contracts or other forms of obligations to pay construction charges shall continue through December 31, 1960.

(b) The Secretary is authorized, subject to the provisions of this subsection to defer the time for the payment of such part of any installments of construction charges under any repayment contract or other form of obligation as he deems necessary to adjust such installments to amounts within the probable ability of the water users to pay. Any such deferment shall be effected only after findings by the Secretary that the installments under consideration probably cannot be paid on their due date without undue burden on the water users, considering the various factors which in the Secretary's judgment bear on the ability of the water users so to pay.

The Secretary may effect the deferments hereunder subject to such conditions and provisions relating to the operation and maintenance of the project involved as he deems to be in the interest of the United States. If, however, any deferments would affect installments to accrue more than twelve months after the action of deferment, they shall be effected only by a formal supplemental contract. Such a contract shall provide by its terms that, it being only an interim solution of the repayment problems dealt with therein, its terms are not, in themselves, to be construed as a criterion of the terms of any amendatory contract that may be negotiated and that any such amendatory contract must be approved by the Congress unless it does not lengthen the repayment period for the project in question beyond that permitted by the laws applicable to that project, involves no reduction in the total amount payable by the water users, and is not in other respects less advantageous to the Government than the existing contract arrangements. The Secretary shall report to the Congress all deferments granted under this subsection.