If you are a US person and required to file a US tax return, you report the gain or loss using the US$ values, regardless of currency fluctuations. If you have a capital loss when viewed in US$, that is what you report, even if you have a gain when viewed in the currency of the other country. Capital losses are generally deductible on your US tax return against other capital gains income, except that capital losses on personal use property are not deductible. (Such as your main or vacation home.)