Certificates of indebtedness must be designated as "Montgomery County self-insurance certificates of indebtedness" and may be sold from time to time and must be sold in the manner as the county executive determines; provided, that the same must not be sold at less than par and accrued interest. (Mont. Co. Code 1965, § 5-5; 1978 L.M.C., ch. 36, § 1; 1986 L.M.C., ch. 44, § 1.)