The Secretary may order a member of the Service (other than a member employed under section
3951 of this title) who is a citizen of the United States to take a leave of absence under section
6305 of title
5 (without regard to the introductory clause of subsection (a) of that section), upon completion by that member of 12 months of continuous service abroad. The Secretary shall order on such a leave of absence a member of the Service (other than a member employed under section
3951 of this title) who is a citizen of the United States as soon as possible after completion by that member of 3 years of continuous service abroad.