Mont. Code Ann. § 19-19-303

A police officer whose service with the city has been discontinued by other than death or placement upon the retired list must be entitled to the return of the amounts paid to the fund through deductions from the officer's salary. If the officer has 10 years or more of service, the amount paid must include regular interest on the amounts. If the officer has less than 10 years of service, the officer must receive only the amount paid through salary deductions, without interest.