For refunds of an order made in a currency different from those available in your PayPal account, you have 2 choices:
Add all the currencies from your store to your PayPal account, in order to avoid rejecting cross-currency transactions or encountering refund issues.
To do this, go to paypal.com > Login > Settings > My Money > Currencies management > Add a currency.
Contact PayPal and request them to activate the cross-currency refund option. ("Contact Us" tab on paypal.com).