22 USC 2513 - Assignment of personnel to foreign governments or international organizations

(a) Authority; oath of allegiance 
In furtherance of the purposes of this chapter, the head of any agency of the United States Government is authorized to detail, assign, or otherwise make available any officer or employee of his agency
(1)  to serve with, or as a member of, the international staff of any international organization, or
(2)  to any office or position to which no compensation is attached with any foreign government or agency thereof: Provided, That such acceptance of such office or position shall in no case involve the taking of an oath of allegiance to another government.
(b) Benefits of detailed personnel 
Any such officer or employee, while so detailed or assigned, shall be considered, for the purpose of preserving his allowances, privileges, rights, seniority, and other benefits as such, an officer or employee of the United States Government and of the agency of the United States Government from which detailed or assigned, and he shall continue to receive compensation, allowances, and benefits from funds authorized by this chapter. He may also receive, under such regulations as the President may prescribe, representation allowances similar to those allowed under section 4085 of this title. The authorization of such allowances and other benefits, and the payment thereof out of any appropriations available therefor, shall be considered as meeting all of the requirements of section 5536 of title 5.

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(c) Reimbursement provisions 
Details or assignments may be made under this section
(1) without reimbursement to the United States Government by the international organization or foreign government;
(2) upon agreement by the international organization or foreign government to reimburse the United States Government for compensation, travel expenses, and allowances, or any part thereof, payable to such officer or employee during the period of assignment or detail in accordance with subsection (b) of this section; and such reimbursement shall be credited to the appropriation, fund, or account utilized for paying such compensation, travel expenses, or allowances, or to the appropriation, fund, or account currently available for such purpose; or
(3) upon an advance of funds, property or services to the United States Government accepted with the approval of the President for specified uses in furtherance of the purposes of this chapter; and funds so advanced may be established as a separate fund in the Treasury of the United States Government, to be available for the specified uses, and to be used for reimbursement of appropriations or direct expenditure subject to the provisions of this chapter, any unexpended balance of such account to be returned to the foreign government or international organization.