31 C.F.R. § 281.3
Foreign exchange collected by agencies shall be delivered promptly into the custody of accountable officers for credit to accounts of the Secretary of the Treasury (hereinafter referred to as the Secretary) unless otherwise directed by the Secretary. The term "collections," for the purpose of these regulations in this part, does not include foreign exchange acquired by the United States by purchase with dollars. The accountable officer shall maintain records, showing the collections, by source, and indicating the miscellaneous receipt accounts or other accounts in the Treasury to be credited with dollar proceeds from sale of the foreign exchange, and such further classifications as may be needed to indicate exchange which can be used only for restricted purposes. Accountable officers shall be advised by the collecting agencies of the source of collections and any restrictions on the use of the foreign exchange in order that the foregoing records may be maintained.
31 C.F.R. §281.3