I am not a specialist, so my opinion can't be considered as correct, but I would assume that property it's like shares. So it has purchase price (converted to functional currency - USD) by the exchange rate on the date of purchase.
https://www.irs.gov/individuals/international-taxpayers/yearly-average-currency-exchange-rates
Also, make sense to read this article to check the correctness of all assumptions.
https://www.irs.gov/forms-pubs/about-publication-527
