7-7-2270. Authority to purchase outstanding bonds. Whenever there is available money in any sinking fund over and above the amount required for payment of the principal and interest becoming due on the next interest payment date and sufficient to pay and redeem one or more outstanding bonds of the issue or series to which the sinking fund belongs and whenever such bonds are not held by the state of Montana and are not subject to redemption, the county treasurer, at the direction of the board of county commissioners, shall purchase such bonds if this can be done at not more than par and accrued interest or at such reasonable premium as the board may feel justified in paying, not in any case exceeding 5%.