Not sure how that would work in your Favor, because either your Credit Card or Paypal will then do the Currency Conversion. That will also be the Case when doing a Bank Transfer for that Matter. So if the Exchange Rate changes between thee 2 Dates (Purchase and Refund), you will get back the amount in the "Local" Currency, whose Exchange Rate changed.
Though, in Europe (in Denmark that's the Case at least), I believe that the Seller doesn't have the right to charge you until they actually ship the Article. Until that happens, they can only place a Money Reservation on your Card, not charge it. Thus the "only" Thing you would lose is that you cannot actually use that Money until they unfreeze it.