Whenever there may be in the interest and sinking fund of any county, city, incorporated town or village, school district, or other political corporation or subdivision of the state, a sufficient amount of money belonging to any interest and sinking fund, which cannot be properly applied to the payment of existing bonds for the payment and redemption of which said fund has been levied and collected, to purchase one or more general obligation bonds, notes, or certificates of the state of Missouri or of the United States, or of any county, city, or school district in this state, the county commission of such county or the governing authority of such city, incorporated town or village, school district, or other political corporation or subdivision of the state may order their treasurer to purchase such bond, note, or certificate of the state of Missouri, or of the United States, or of any county, city or school district in this state, limiting him as to kind, terms and conditions of such purchase; provided, that no such bond or securities shall be ordered to be purchased, and maturity date of which shall extend beyond the maturity date of the bonds for which such sinking fund is provided and intended to pay; provided further, that other bonds, notes, certificates and securities than those herein mentioned may be purchased when so expressly authorized by law.