During the construction of said improvement said commissioners shall have the right to allow payment thereof, in installments as the work progresses, in proportion to the amount of work completed: PROVIDED, That no allowance or payment shall be made for said work to any contractor or subcontractor to exceed seventy-five percent of the proportionate amount of the work completed by such contractor or subcontractor, and twenty-five percent of the contract price shall be reserved at all times by said board of commissioners until said work is wholly completed, and shall not be paid upon the completion of said work until ninety days have expired for the presentation of all claims for labor performed and materials, goods, wares, merchandise and provisions furnished or used in the construction of said improvements; and upon the completion of said work and the payment of all claims hereinbefore provided for according to the terms and conditions of said contract, said commissioners shall accept said improvement and pay the contract price therefor.