Operate allows you to choose a fixed exchange rate that applies to specific customers. If you have agreed on such a rate with your customer, you can configure it under Settings > General > Organizations > Account Fixed Exchange Rate.
All you need to do to set this up is:
Choose the customer account to which the fixed exchange rate is applied.
Select the currency code in relation to which the exchange is being done.
Select a Start and End Date between which the fixed exchange rate is being applied.
Add the exchange rate.
Make sure to click Save to apply your changes.