I don't think it is because your are exchanging one currency to another and you have the possibility to benefit from the volatility that day. The volaitily is when you are relying on the bank, or how well the economy is performing. It is almost like investing in a stock (for a permissable company i.e. not alocholic, wine company). If the company does well, that just means you are helping and supporting them. Of course when you benefit from the bank it means, you earn interest income, you can gain, but that gain I think is halal because it is not coming from a haram company, but then again we don't who those companies are. I think I need to look this up myself..sorry I wasn't much help.
walakum salaam