Any city officer or employee who has a substantial financial interest, direct or indirect, or by reason of ownership of stock in the corporation, in any contract with the City or in the sale or purchase of any land, material, supplies, or services to the City or to a contractor supplying the City, as may subsequently be defined by the Council, shall make known that interest and shall refrain from voting upon or otherwise participating in that capacity as a city officer or employee in the making of such sale or purchase, or in the making or performance of such contract. Any city officer or employee who willfully conceals such a substantial financial interest or willfully violates the requirements of this section shall be guilty of malfeasance in office or position and shall forfeit the office or position. Violation of this section with the knowledge express or implied of the person or corporation contracting with or making a sale to the City shall render the contract or sale voidable by the Manager or the Council.