28 C.F.R. § 21.3
(a) Aliens entitled to payment of $30 per day. The following aliens are entitled to witness fees and allowances provided in § 21.4:
(b) Aliens entitled to payment of $1 per day. An alien who is “excludable” in accordance with 8 U.S.C. 1226, but whose removal is stayed by the Attorney General (in accordance with 8 U.S.C. 1227(d)) because:
(2) The testimony of the alien is necessary on behalf of an indigent criminal defendant in accordance with Rule 17(b) of the Federal Rules of Criminal Procedures,
is entitled to a $1 per day witness fee. No other fees and allowances are authorized.