That's strange. My experience is that the rewards card acts more like a gift card than a credit card. The purchase amount was deducted right away from the card balance. The receipt will show your balance when you use it for a purchase. I've used my card multiple times a day during a wdw trip. For example, used it to pay for breakfast, then to buy something at a shop, then buy a Mickey bar. The purchases came off the remaining balance immediately.
Maybe for some reason the purchase did not process properly. Are you still at the park? Guest relations at the parks are listed as places you can get a redemption card. Maybe they can help you.