26 U.S. Code § 7457 - Witness fees
Any witness summoned or whose deposition is taken under section 7456 shall receive the same fees and mileage as witnesses in courts of the United States.
Such fees and mileage and the expenses of taking any such deposition shall be paid as follows:
(1) Witnesses for Secretary
In the case of witnesses for the Secretary, such payments shall be made by the Secretary out of any moneys appropriated for the collection of internal revenue taxes, and may be made in advance.