Any city officer or employee who has a substantial financial interest, either direct or indirect in any contract with the city or in the sale of any land, material, supplies or services to the city or to a contractor supplying the city shall make known that interest and shall refrain from voting upon or otherwise participating in his capacity as a city officer or employee in the making of such sale 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 his office or position. Violation of this section with the knowledge expressed 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 Common Council.