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You can set the currency of deals using the Currency property on your deal records. The Amount will then appear in the selected currency. Based on the exchange rate you've set for the currency, the amount will then be converted to your company currency in the Amount in company currency property
By adding the following formula field to your object, you leverage built-in Salesforce functionality. You would need to substitute your field name into the formula below (in place of “Amount”), and then drop the formula field into your S-Docs template. You do not need to add any of these fields to any page layout
If you have any kind of price/amount field on a Contract, once the Contract has been saved you cannot edit the currency.  On the Contract object there is not separate currency field created, it is tied to the value you enter. So you can edit the amount, but if you make an error choosing the currency you can't amend it. When you create a custom object and add the currency field it creates a separate, editable field for the currency