If you wish to create invoices or estimates in a different currency than the base currency, you will need to associate the currency to the customer. The instructions to associate a currency to your customer, is given in this link.
However, once you record transactions to the customer in the currency you have associated them with, you will not be able to change the currency. So, if you have associated a contact in a different currency other than the base currency(say USD) and recorded transactions for this customer, you won't be able to create invoices or estimates in any other currencies.
I hope this helps.
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