(a) A post differential may be granted on the basis of conditions of environment which differ substantially from conditions of environment in the continental United States and warrant additional pay as a recruitment and retention incentive. A post differential may be granted to an employee officially stationed in the United States who is on extended detail in a foreign area. A post differential under this subsection may not exceed 35 percent of the rate of basic pay.
(b) Any employee granted a differential under subsection (a) of this section may be granted an additional differential for an assignment to a post determined to have especially adverse conditions of environment which warrant additional pay as a recruitment and retention incentive for the filling of positions at that post. An additional differential for any employee under this subsection-
(1) may be paid for each assignment to a post determined to have such conditions;
(2) may be paid periodically or in a lump sum; and
(3) may not exceed 15 percent of the rate of basic pay of that employee for the period served under that assignment.