Mont. Code Ann. § 25-14-312

Whenever a prisoner is committed to jail on an execution issued on a judgment recovered in a civil action, the creditor or the creditor's agent or attorney shall advance to the jailer, on the commitment, sufficient money for the support of the prisoner for 1 week and shall make the same advance for every successive week of imprisonment, and in case of failure to do so, the jailer shall discharge the prisoner from custody, and the discharge has the same effect as if made by order of the creditor.