When the testimony of a person who is serving a sentence, is in pretrial detention, or is otherwise being held in custody, in a foreign country, is needed in a State or Federal criminal proceeding, the Attorney General shall, when he deems it appropriate in the exercise of his discretion, have the authority to request the temporary transfer of that person to the United States for the purposes of giving such testimony, to transport such person to the United States in custody, to maintain the custody of such person while he is in the United States, and to return such person to the foreign country.