Small operators tend to be sloppy as they tend to focus more on food and personal rapport but they stand the risk of lawsuits or complaints if they happen to meet some less tolerant customers especially if they notice that the vendors are not ignorant people. I would feel cheated over a dollar but would gladly take it back as a refund and drop further actions if the interactions are cordial. In this case as it turns out, the vendor profited from the free advertisement from the professor who praised the food.