SalesAmountSelectedCurrency

This dax will be more useful if there are more values(all currencies across the globe) in a CurrencyAlternateKey.

SalesAmountSelectedCurrency = 
VAR SelectedCurrency = SELECTEDVALUE(dim_Currency[CurrencyAlternateKey])
RETURN
IF(
    NOT ISBLANK(SelectedCurrency),
    CALCULATE(
        SUM(fact_InternetSales[SalesAmount]),
        dim_Currency[CurrencyAlternateKey] = SelectedCurrency
    ),
    "No Currency Selected"
)