When there is an agreement between a district and the United States of America, or any department or agency thereof, which provides for the issuance of refunding bonds of the district aggregating in principal amount a sum less than the principal amount of the bonds to be refunded and for the creation of a reserve fund to pay installments of principal or interest of the refunding bonds when the amount in the bond fund is insufficient to pay the installments, and when the unpaid principal amount of the assessment securing the bonds to be refunded is, or upon the carrying out of the plan for refunding provided in the agreement will be, in excess of the principal amount of bonds

secured by the assessment, the board may declare by resolution that all or part of the excess of the assessment shall be called from time to time for the purpose of creating a reserve fund.