This was a business event (your employer undoubtedly had a reason to purchase a table at this event, if only for community PR) and your employer purchased the tickets. I can't think of any reason why your employer can't insist that the prize money belongs to them.
It would be awfully nice if they congratulated you on your good luck and let you keep the money but they don't have to do that.
A person, who is nice to you, but rude to a waiter, is not a nice person. (This is very important. Pay attention. It never fails.)