Prior to the transfer of an offender to the United States, the fact that the offender consents to such transfer and that such consent is voluntary and with full knowledge of the consequences thereof, shall be verified in the country in which the sentence was imposed by a United States magistrate judge, or by a citizen specifically designated by a judge of the United States as defined in section 451 of title 28, United States Code. The designation of a citizen who is an employee or officer of a department or agency of the United States shall be with the approval of the head of that department or agency.