5 USC 5927 - Advances of pay

(a) Up to three months pay may be paid in advance
(1) to an employee upon the assignment of the employee to a post in a foreign area;
(2) to an employee, other than an employee appointed under section 303 of the Foreign Service Act of 1980 (and employed under section 311 of such Act), who
(A) is a citizen of the United States;

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(B) is officially stationed or located outside the United States pursuant to Government authorization; and
(C) requires (or has a family member who requires) medical treatment outside the United States, in circumstances specified by the President in regulations; and
(3) to an employee compensated pursuant to section 408 of the Foreign Service Act of 1980, who
(A) pursuant to United States Government authorization is located outside the country of employment; and
(B) requires medical treatment outside the country of employment in circumstances specified by the President in regulations.
(b) For the purpose of this section, the term country of employment, as used with respect to an individual under subsection (a)(3), means the country (or other area) outside the United States where such individual is hired (as described in subsection (a)(3)) by the Government.