In Zoho Books, the item prices will always be saved in the base currency.
To invoice a customer in a foreign currency, you’ll have to associate that currency with the customer. This will be reflected when you raise an invoice for that particular customer and the item price in your base currency will be converted to your customer’s currency based on the exchange rate.
Notes: Only one currency can be associated to a contact
To associate a currency to your customer:
Books
Simplify accounting
and GST filing.