Quick search by citation:

22 U.S. Code § 3974 - Border equalization pay adjustment

(a) In general

An employee who regularly commutes from the employee’s place of residence in the continental United States to an official duty station in Canada or Mexico shall receive a border equalization pay adjustment equal to the amount of comparability payments under section 5304 of title 5 that the employee would receive if the employee were assigned to an official duty station within the United States locality pay area closest to the employee’s official duty station.

(b) Employee definedFor purposes of this section, the term “employee” means a person who—
is an “employee” as defined under section 2105 of title 5; and
is employed by the Department of State, the United States Agency for International Development, or the International Joint Commission of the United States and Canada (established under Article VII of the treaty signed January 11, 1909) (36 Stat. 2448), except that the term shall not include members of the Service (as specified in section 3903 of this title).
(c) Treatment as basic pay

An equalization pay adjustment paid under this section shall be considered to be part of basic pay for the same purposes for which comparability payments are considered to be part of basic pay under section 5304 of title 5.

(d) Regulations

The heads of the agencies referred to in subsection (b)(2) may prescribe regulations to carry out this section.