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.

Where the transfer to the United States of a person in custody for the purposes of giving testimony is provided for by treaty or convention, by this section, or both, that person shall be returned to the foreign country from which he is transferred. In no event shall the return of such person require any request for extradition or extradition proceedings, or proceedings under the immigration laws.

Where there is a treaty or convention between the United States and the foreign country in which the witness is being held in custody which provides for the transfer, custody and return of such witnesses, the terms and conditions of that treaty shall apply. Where there is no such treaty or convention, the Attorney General may exercise the authority described in paragraph (a) if both the foreign country and the witness give their consent.