Here's how it works.
SalesEach sale can have its own currency, separate from your system currency. All values for the sale will appear, both on screen and in reports, in the chosen currency.
The detail pane of the Sales library has a Currency field, which displays the chosen currency for each sale, or "Default" for sales using your system currency:
Click the Choose currency button to the right of the field to select a currency. The Choose currency sheet displays the currencies you use the most at the top of the list: