If you're in the United States, "Use the exchange rate prevailing when you receive, pay, or accrue the item." http://www.irs.gov/businesses/small/article/0,,id

If you're in the United States, "Use the exchange rate prevailing when you receive, pay, or accrue the item." http://www.irs.gov/businesses/small/article/0,,id=130524,00.html http://www.irs.gov/businesses/small/article/0,,id=130524,00....