In general, lowercase the names of currencies, but capitalize the reference to the country or region.
Hong Kong SAR dollar
Specific monetary amounts
When you’re referencing specific amounts of money, use the currency code, followed by the amount, with no space.
The company generated BRL2.89 billion (USD1.42 billion) in net revenue in 2015.
If it’s clear which currency you mean, it’s OK to use just the symbol. When referring to a specific amount in euros, use the euro symbol (€), not the word euros.
One of the largest companies in the United States, Adatum Corporation generated $1.42 billion in net revenue in 2015.
Adatum Corporation generated €1.42 billion in net revenue in 2015.