There is no setting in the Dashboard for a customer's default currency. The default currency for a customer is set automatically when any of the following actions are taken for the first time:
The customer is subscribed to a plan
An invoice is created for the customer
An invoice item is created for the customer
The customer’s balance is adjusted
A flat-rate coupon is attached to the customer (not a percent-off coupon)
Once any of the actions above are taken for a customer and the default currency is set, the currency cannot be changed.
The only way to change the currency you charge a customer in is to create a new
Customer object representing that customer, attaching a payment method, and then pursuing the desired task in the right currency.
Once a customer's currency is set, you cannot create a subscription, invoice, invoice item, balance adjustment, or coupon in another currency for that customer.