The majority of the fee was paid up front. However the trigger point for an additional payment based on appearances has not yet been met and neither, of course , has the further payment due if Hull stay up. So he will be in Hull's books at a cost of around £3m and will be shown as a sale of £3m in Cardiff's books ( the " conditional" further payments are not shown as a profit in a club's books until those conditions are met).
I can't see the club being willing to pay out £3m on a goalie( or any other player for that matter) or Hull being willing to take a big loss on a player they only signed a few months ago, but a loan move may not be out of the question