When, in the making of any improvement in a district, bonds or warrants of such districts are sold for the purpose of providing funds for such improvement, any money is left from the proceeds of the sale after the improvement has been paid for, then in such cases the board of directors of such drainage district or drainage and levee district, with the approval of the county court of the county having jurisdiction of the cause, has the power to dispose of such surplus money by using such surplus funds for improving, if needed, the main improvement of the district, or for opening, making or improving such lateral drains or ditches in the district as may be deemed best, or such surplus funds and money may be used in paying all or in part any assessment made on the lands of the district and not yet collected, in which event such particular assessment, or part thereof, need not be collected. Alternately, such surplus of funds may be used in purchasing and retiring any of the bonds or warrants issued; provided, that they can be bought at not above par with any accrued interest on the bonds or warrants. The board of directors with the approval of the county court has the power to dispose of such surplus funds in any of the methods above set out that they and the court may deem best for such district, the court to concur, by its order or decree, in such disposition.