If you charged your customer in a currency different from you Duffel settlement currency, the payment amount was converted to your settlement currency using the mid market rate for that day of the transaction.
When a refund is issued the same conversion will happen but in reverse order, from your settlement currency to the payment currency.
This conversion will be done using the mid-market rate for the day the refund is issued. This rate can be higher or lower than the refund used at the time of payment. It is you responsibility account for these differences in how you price your products & services.