Magento adds coupon discount amount to the total price with Barclaycard ePDQ payment

I have a Magento shop and we use Barclaycard ePDQ as our payment method. I recently created a new cart rule that gives you 10% off discount when using a coupon code. Everything goes well with Magento - the coupon applies - bu开发者_JAVA百科t after I press Checkout and it takes me to the Barclaycard ePDQ page, I notice that the total amount to pay is actually the regular price + the discount amount, which is really weird because when I look at my order reports it seems that the grand total is with the discount.

I saw this problem in an earlier version of Magento with regards to Paypal. The issue was that the processor was expecting a negative value (-5.00) and Magento was passing a positive one (5.00) or vice versa. I'd check the redirect page from Magento and see if the discount is applied with the opposite sign.