No animal showing physical evidence of tuberculosis or brucellosis, or in which such disease shall have been indicated as a result of the tuberculin or other approved test, shall be sold other than for immediate slaughter, said slaughtering to be ordered by and under the supervision of the commissioner or his agent, except under a written contract approved by the commissioner, signed by both parties, describing the animal and stating that it is believed to be tuberculous or infected with brucellosis. A contract of sale as provided by this section shall be executed in triplicate and one copy thereof delivered to the purchaser, one kept by the seller, and the other delivered to the commissioner.