When referring to specific amounts of money, always use the relevant currency symbol, rather than spelling out “dollars,” “euros,” “pounds,” or whatever. When referring to the local unit of currency, spell out “dollar,” “euro,” “pound,” and so on.
Many countries use the same word for their unit of currency. For instance, the US, Canada, and Australia all use “dollars.” When confusion is likely, follow the dollar amount with the three-letter currency code from the International Organization for Standardization.
For large sums of money (over $1 million), use numerals and spell out “million,” “billion,” and so on, rounding off to one decimal place where appropriate. Where space dictates, feel free to abbreviate “$5.2 million” to “$5.2m.”
When writing a full dollar amount (or whichever currency is applicable), it’s best to leave off the decimal amount.