When you add a Payment Method to Sling Money, you will be asked what currency your underlying account is in. Sling Money will always charge your Payment Method in this currency, regardless of the Display Currency used to denominate a transaction. You can change the currency of a Payment Method at any time.
For example, if your display currency is in Euro (EUR), but your payment method was set in GBP, if any fee will be deducted, it will be in GBP.