if paypal standrd, there will be a hidden element on the conform page that sets the currency.
<input type="hidden" name="currency_code" id="currency_code" value="USD" />
if using payments pro, there will be code need the top of the confirm page that setts the currency:
$PP_DirectPayment_required[0][$nextIndex] = "Currency";
$PP_DirectPayment_required[1][$nextIndex] = "USD";