For the purpose of carrying into effect provisions contained in any contract so to be executed, the board of such district may, with the approval of the electors of the district, issue bonds in any denomination, bearing interest from date at a rate determined by the board, and to mature in not more than 40 years. The bonds shall be general obligations of the district and shall be paid by the revenue derived from the annual charges or assessments of the district, which shall be made in accordance with the terms of the contract. All of the owners of the real property within the district shall be liable to be charged or assessed for such payments; provided, however, that the contract may contain a provision permitting the release of any land in the district from the lien to secure the payment of such bonds, or relieving any lands in the district from any obligation to pay any assessments thereafter levied for the purpose of paying the bonds or the interest accruing thereon, by payment to the district of an amount provided in the contract. [Amended by 1981 c.94 § 48; 1991 c.459 § 426]