All bonds of the county held by the sinking fund commission shall be stamped in a conspicuous manner to show that they have been purchased for this purpose. They shall never be reissued or sold. The sinking fund commission shall not require the county to pay interest on any of its bonds held by the commission, unless the commission deems the payment thereof necessary for the purpose of having adequate funds on hand to pay the bonds of said county as they mature and become payable. All bonds of the county held by the commission shall be cancelled immediately upon their maturity.
16 P.S. § 4992