After the completion and acceptance of the improvement or improvements made under the Combined Improvement Act, the city or village may issue and sell its negotiable coupon bonds to be known as public improvement bonds in an amount not exceeding the balance of the unpaid cost of the improvement or improvements. The bonds shall be payable in not to exceed twenty years from date and bear interest payable annually or semiannually. All money collected from the special assessments shall be placed in a sinking fund to pay the cost of the improvement or improvements and the bonds issued under the Combined Improvement Act.