Mont. Code § 85-7-2027
The county treasurer shall place the proceeds of the sale to the credit of the district, and the proceeds may be paid out by the county treasurer only upon the written order of the board of commissioners, signed by the president and secretary under the seal of the district. The proceeds may be spent only for the purpose for which the bonds were issued. If any portion of the funds realized from the sale of bonds is not needed immediately for the purpose for which the bonds were issued, the board of commissioners may direct the investment of the funds and any other surplus funds of the district or any portion of the funds in legal investments backed, insured, or guaranteed by the United States or the state of Montana, including federal and state agency obligations. The county treasurer shall transfer to the credit of the district and place to the credit of the fund or funds, as the board of commissioners may direct, all interest received upon investments of the district entrusted to the treasurer's care.
§ 85-7-2027, MCA