Mont. Code § 7-3-4370
Whenever any city organized under this part and part 44 includes the county seat of the county in which it is situated, any unused space in the county buildings in such city may be rented to the city commissioners for municipal use by the board of county commissioners for such rent as shall represent an income of not more than 6% upon the investment in such buildings proportionate to the space rented. Such commissioners may also contract with the board of county commissioners for the performance by county officials or employees of any kind of municipal work which can be feasibly performed by them. The compensation for such work shall be based upon additional cost to the county of its performance, and such compensation shall be paid into the general fund of such county unless otherwise provided by law.
§ 7-3-4370, MCA