Invoices of all supplies of whatever kind shall be furnished in duplicate by the contractor or seller at the time of every delivery of said supplies, 1 of which shall be transmitted to the department or institution head to which supplies are sent, and 1 by the same mail to the purchasing agent. As soon as the supplies have been received and examined by the head of the institution or department to which the same shall have been shipped, and if he shall find them to correspond in every particular with the invoices transmitted him and the samples by which the supplies were sold, he shall transmit to the purchasing agent the said invoice with a certificate thereon that the supplies received correspond in every particular with the invoice and the sample by which the supplies were sold, and if the purchasing agent shall, upon further examination find such invoice to be correct, he shall transmit it with his approval to the board of supervisors or board of auditors, and when such invoice so approved by the head of the institution or department to which the supplies named therein have been furnished and by the purchasing agent, shall have been further approved by the board of supervisors or auditors it shall be the duty of the board to order their warrant for the amount due on the invoice, or upon so much thereof as has been allowed, upon the county treasurer. Whenever the department head shall reject any supplies, he shall immediately notify the purchasing agent and in such notification shall give his reasons for the rejection. All supplies shall be at the risk of the contractor or seller until accepted by the purchasing agent. Whenever any supplies shall be rejected, it shall be the duty of the contractor or seller to immediately take same away and replace such supplies with supplies that will conform to the provisions of the contract.