In order to facilitate the extinguishment of the county debt, the county bonds purchased by the commission from time to time shall be paid off and cancelled according to the priority of their maturity. The commission may, at its discretion, withhold the purchase of such maturing county bonds until after those purchased from a later issue of county bonds shall be paid off and cancelled. In such an event, if it appears that there will not be sufficient funds in the hands of the commission to meet the payment of such earlier maturing bonds, the same shall then be sold by said commission at not less than par.
16 P.S. § 4993