1. The contract with the United States may also call for the payment of the amount or any part thereof to be paid by the district to the United States at such times and in such installments and with such interest charges, which must not exceed by more than 5 percent the Index of Revenue Bonds which is in effect at the time the agreement is made, as may be agreed upon, and for assessment and levy therefor as provided in this chapter.
2. The obligations of those contracts are a prior lien to any subsequent bond issue.