Forex Tax Research
Section 988.–Treatment of Certain Foreign Currency Transactions
An instrument that requires payments to be made in a foreign currency (that is, nonfunctional currency) can be debt for U.S. federal income tax purposes.
Original source from irs.gov: https://www.irs.gov/pub/irs-drop/rr-08-1.pdf
Currency exchange rates
The Internal Revenue Service has no official exchange rate. Generally, it accepts any posted exchange rate that is used consistently.
When valuing currency of a foreign country that uses multiple exchange rates, use the rate that applies to your specific facts and circumstances.
Note: The exchange rates referenced on this page do not apply when making payments of U.S. taxes to the IRS. If the IRS receives U.S. tax payments in a foreign currency, the exchange rate used by the IRS to convert the foreign currency into U.S. dollars is based on the date the foreign currency is converted to U.S. dollars by the bank processing the payment, not the date the foreign currency payment is received by the IRS.