(a) Witnesses (whether appearing voluntarily, or under a subpoena) shall be paid the fee and mileage allowances which are paid subpoenaed witnesses in the courts of the United States: Provided, That any witness who is employed by the Federal Government shall not be entitled to receive witness fees in addition to compensation received pursuant to Sec. 1429.13.
(b) Witness fees and mileage allowances shall be paid by the party at whose instance the witnesses appear, except when the witness receives compensation pursuant to (the preceding section).