The contract provisions for the payment of construction charges to the United States, and the bonds securing the payment of the same, if any are issued and deposited, may be of such denomination and may call for the payment of such interest not exceeding six percent per annum, may provide for such installments and for repayment of the principal at such times, as may be required by the federal laws and as may be agreed upon between the board and the appropriate federal agency.
ORS 553.635