Discovery Card very unhelpful

I booked return tickets on Delta Airways using my Discovery Card, and received the booking confirmation via email (R37 532). The problem is that the Discovery Card statement shows a second amount was taken (39 111.78), and a much lessor refund was done (37 671.66). In total I have been left with approximately R2500 less. <br> <br> I phoned Delta and they have no record of the second transaction and refund. They only have on record the first correct transaction. Discovery card has been very unhelpful, attributing this R2500 loss to \exchange rates"and that I should \""discuss this with Delta\"". This explanation is unacceptable"