85-7-2140. Investment of funds. The board of commissioners shall direct the investment of the funds in any bond sinking fund in legal investments backed, insured, or guaranteed by the United States or the state of Montana, including federal and state agency obligations, whenever in their judgment the investment may be to the best interest of the district. All such investments shall be converted into cash in time to meet the principal on the bonds payable from such sinking fund promptly at their maturity.