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.