The board of directors of any irrigation district in the State of Nebraska, if it considers it for the best interest of such district, shall have the power to provide a sinking fund with which to pay and retire outstanding bonds of the district. For the purpose of creating, establishing, and maintaining such fund, such board may levy a tax each year of not to exceed eighty-seven and five-tenths cents on each one hundred dollars upon the taxable value of all the taxable property in such district as fixed by the district assessor. Following such levy, the board may by contract with the owners of such bonds pay and retire any bonds of the district and interest accrued thereon, whether such bonds are due and payable or not.