Each witness who appears before the department of transportation, by order of the department of transportation, shall receive for attendance the compensation provided by law, which shall be paid by the state treasurer on warrant of the commissioner of finance and administration, upon the presentation of proper voucher sworn to by such witness, and approved by the commissioner of transportation; provided, that no witness shall be entitled to any witness fees or mileage who is directly or indirectly interested in any such company in this state or out of it, or who is in any way interested in any stock, bond, mortgage, security, or earnings of any such company, or who shall be the agent or employee of such company, or an officer thereof when summoned at the instance of such company. No witness furnished with free transportation shall receive compensation for the distance traveled on such free transportation.