The sum to which a witness is entitled shall be paid out of the state treasury in any case of attendance before either house or a committee of the General Assembly and in any other case in which the attendance is for the Commonwealth except when it is otherwise specially provided. In all other cases it shall be paid by the party for whom the summons issued. The payment shall be on a certificate of the person required by § 17.1-612 to make the entry or the clerk of the court in whose minutes the entry is made. The certificate shall express by letters and not by figures the separate amount to which the witness is entitled for his attendance, traveling, and tolls which he may have to pay and the aggregate thereof. No clerk or other person authorized to make such entry or give such certificate shall become interested by purchase in any claim payable out of the state treasury which by law he is authorized to certify.