I can think of five issues relating to how you state amounts of money in a contract.
Words and Numerals
Drafters will often do the words-and-numerals thing when stating amounts of money: Acme shall pay Widgetco One Million Dollars ($1,000,000). The idea is that whereas numerals are easier to read than words, they’re more prone to typographic errors; adding words provides a safety net, as courts generally hold that if there’s a discrepancy between words and numerals, the words govern.
But stating numbers in words and numerals distracts the reader and represents another possible source of inconsistency, yet adds little or nothing of value in return. If your proofreading isn’t up to making sure that when you say 12 you really mean 12, the words-and-numerals system isn’t going to save you.
Generally, I recommend that you use words for numbers up to ten and numerals thereafter. (MSCD 10.4 mentions some obvious exceptions.) The Chicago Manual of Style 9.23 recommends that you use for stating amounts of money the same approach you use for numbers generally. But for purposes of contracts, I recommend that you state all amounts of money using only numerals. In a commercial context, it would be very odd to use words for nine dollars but numerals for $12.
If you’re dealing with a big number in a sensitive context—if, for example, you’re stating in a promissory note the principal amount of the loan—and using both words and numerals makes you feel more secure, then go ahead. Indulging in words-and-numerals every so often won’t hurt anyone.
The Chicago Manual of Style 9.24 says that when stating amounts of money you should use zeros after a decimal point only when fractional amounts appear in the same context. In other words, say $2 rather than $2.00, unless the provision in question also refers to $3.78 and $12.93. I suspect that most U.S. drafters would use $2.00 whatever the context, but I’m with CMOS on this.
If you’re stating less than a whole unit of currency, put a zero before the decimal point, as in $0.32.
Very Large Amounts
It’s commonplace for drafters to use a mixture of words and numerals to express large amounts of money: $4.4 billion; $3 million. This approach can make make amounts of money easier to read, particularly if a given provision includes several large amounts. But don’t switch to a mxiture of words and numerals if other numbers in the same context use all numerals. And don’t go beyond one decimal point to express fractional amounts.
Any reference to an amount of money would have to specify the currency. You can accomplish that by using a currency sign or by using the three-letter currency code specified for that currency in ISO 4217, the list established by the International Standards Organization. For example, the currency sign for the Euro is €; the currency code is EUR. And the currency sign for the Norwegian Kroner is Kr; the currency code is NOK. Which choice is preferable is a function of custom.
Some currency signs are shared by many currencies, and you can make it clear which currency you’re referring to by supplementing the currency sign. For example, the dollar sign ($) is used for currencies in many countries other than the U.S., principally those using currencies denominated in dollars (including Australia, Brunei, and Canada) or pesos (including Argentina, Chile, and Mexico). If a contract between parties from different countries refers to a currency that uses the dollar sign, using a currency code or a suitably modified currency sign, such as A$ for Australian dollars and Mex$ for Mexican pesos, would make it clear which currency is being referred to.
In many countries, especially those in the English-speaking world, the practice is to place the currency sign before the amount. In other countries, it’s placed after the amount. I’d be inclined to always put it before, whatever the currency.
Always put a space between the currency code and the numeral: EUR 2,400,000. If a currency sign consists of one or more letters, put a space between the currency sign and the numeral: SFr. 334,583. But if the currency sign consists of a symbol, don’t use a space, even if you add one or more letters in front of the symbol: C$655,000.
Provision Specifying Drafting Conventions
An alternative way to make it clear what currency is being referred to is to use a provision specifying drafting conventions, such as A$ means Australian dollars. This is one of the few such provisions that does something other than state the obvious.
You could conceivably use such a provision as an excuse not to modify a multi-currency sign. You could, for example, in a given contract use a simple dollar sign in referring to Australian dollars and say $ means Australian dollars. But it would be best to add the extra letter every time you state an amount of money in that contract, as that by itself should be enough to inform most readers what currency is being referred to.