The county executive may negotiate with other units of governments with respect to cooperative purchasing plans and interlocal agreements to purchase when the best interests of the county would be served thereby; PROVIDED, That each of the participating entity shall be separately invoiced by the vendors for purchases made under such plans, and the county of Snohomish shall not be obligated for any purchases other than those required for its own use and supplied pursuant to its order. Such plans or agreements shall be approved by the council. (Added by Ord. 80-110 § 2, Jan. 20, 1981; Amended by Ord. 86-003 § 26, Feb. 12, 1986; Amended by Ord. 92-085, Aug. 12, 1992).