Pub. L. 86-707, tit. II, pt. D, sec. 231

Sec. 231. 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 compensation as a recruitment and retention incentive. Such differential also may lie granted to any employee who is officially stationed in the United States and who is on extended detail in a foreign area. Additional compensation paid as a post differential shall not in any instance exceed 25 per centum of the rate of basic compensation.