Whenever a contract is made for the construction, alteration, or repair of a Foreign Service building which requires payments in a foreign currency, the Secretary of State is authorized to purchase such currency at such times and in such amounts (within the total amount of the payments to be made under such contract) as he may deem necessary, the currency so purchased to be disbursed and accounted for at its cost price.
22 U.S. Code § 294a - Contracts requiring payment in foreign currency
(May 14, 1940, ch. 189, title I, 54 Stat. 186.)
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