For the purpose of making partial payments as the work progresses in an extension district created pursuant to section 19-2402, warrants may be issued by the mayor and city council or the chairperson and village board of trustees, as the case may be, upon certificates of the engineer in charge showing the amount of work completed and materials necessarily purchased and delivered for the orderly and proper continuation of the project, in a sum not exceeding ninety-five percent of the cost thereof and upon the completion and acceptance of the work issue a final warrant for the balance due the contractor, which warrants shall be redeemed and paid upon the sale of the bonds issued and sold as provided in section 19-2405, and which shall bear interest at such rate as the mayor and city council or chairperson and village board of trustees shall order. The city or village shall pay to the contractor interest at the rate of eight percent per annum on the amounts due on partial and final payments beginning forty-five days after the certification of the amounts due by the engineer in charge and approval by the city council or village board of trustees, and running until the date that the warrant is tendered to the contractor. Such warrants shall be registered in the manner provided for the registration of other warrants and called and paid whenever there are funds available for that purpose in the manner provided for the calling and paying of other warrants. For the purpose of paying such warrants and the interest thereon from the time of their registration until paid, the special assessments as provided in section 19-2404 shall be kept as they are paid and collected in a fund to be designated as the sewer and water service extension fund.