22 CFR §1429.14

1. (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 § 1429.13.
2. (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).