No City official or employee shall be directly or indirectly interested in any contract with said City for an amount in excess of $500.00, or furnish any material, or perform any labor, except in the discharge of his or her official duties for which said City officer shall receive a sum in excess of $500.00, unless such contract shall have been awarded upon bids advertised for by publications [which] must have appeared six days previous to the opening of such bids.