the detail to the United Nations, under such terms and conditions as the President shall determine, of personnel of the armed forces of the United States to serve as observers, guards, or in any non-combatant capacity, but in no event shall more than a total of one thousand of such personnel be so detailed at any one time: Provided, That while so detailed, such personnel shall be considered for all purposes as acting in the line of duty, including the receipt of pay and allowances as personnel of the armed forces of the United States, credit for longevity and retirement, and all other perquisites appertaining to such duty: Provided further, That upon authorization or approval by the President, such personnel may accept directly from the United Nations (a) any or all of the allowances or perquisites to which they are entitled under the first proviso hereof, and (b) extraordinary expenses and perquisites incident to such detail;

the furnishing of facilities, services, or other assistance and the loan of the agreed fair share of the United States of any supplies and equipment to the United Nations by the Department of Defense, under such terms and conditions as the President shall determine;

the obligation, insofar as necessary to carry out the purposes of clauses (1) and (2) of this subsection, of any funds appropriated to the Department of Defense or any department therein, the procurement of such personnel, supplies, equipment, facilities, services, or other assistance as may be made available in accordance with the request of the United Nations, and the replacement of such items, when necessary, where they are furnished from stocks.