Incomplete order and unhelpful manager

Last night we purchased a Zinger crunch burger (with extra Zinger sauce) and a Streetwise One (with mash and gravy) at the drive-thru and paid by card. At the time of placing the order, we were told that we need to wait 2 minutes, which then turned into about 7 minutes. On arriving home we discovered that the burger did not have any zinger sauce and the streetwise one had no mash and gravy. When I phoned the branch today, I was advised by the manager that without a proof of purchase they can't do anything in this regard. He was very unhelpful and matter of fact about it, not even apologizing for the error. I told him that I don't have the slip, I mean, who keeps their KFC slips? I told him that we paid by card and surely they should be able to track it from there and he just said no. It is terrible that the manager has the attitude of the shop is right and the customer is wrong unless they are able to proof that the restaurant was at fault. We are regulars and buy from them at least twice a week, but I don't think they will see us going there again.