If some menu item is bad enough that you can't enjoy it, you should send it back after the first bite. At that point, it should be taken off your bill and replaced. If you eat it and then complain at the end of the visit, you are not entitled to having it taken off your bill.
If they offer to take care of it on your next visit, I wouldn't return. That's just a dodge to get you and your party to come back again and spend a few hundred more dollars and they will comp your meal (if the same guy is there and remembers you).