Mont. Code § 7-5-2306
When the amount to be paid as the purchase price for any vehicle or road machinery of any kind, for any other machinery, apparatus, appliance, or equipment, or for any materials or supplies of any kind exceeds $4,000, the county governing body may provide for the payment of the purchase price in installments extending over a period of not more than 10 years, provided that at the time of entering into the agreement for the purchase, there is an unexpended balance of appropriation in the budget for the then-current fiscal year available and sufficient to meet and take care of the portion of the purchase price as is payable during the then-current fiscal year and the budget for each following year in which any portion of the purchase price is to be paid contains an appropriation for the purpose of paying the purchase price.
§ 7-5-2306, MCA