How does he know that it is racism? Could someone who looked just like him have stolen a book from the table and the sales person mistook him for that guy? Did he complain to the manager? You seem to imply that the manager making 100K a year is somehow morally superior to the sales clerk? Why? What managers are always more honest than sales clerks? Really?
Racism is alive and well in this country and everywhere in the world and always will be. There will always be individuals that will judge you because of how you look. The difference is that now racism is an equal opportunity vice. Those with dark skin look askance at those with lighter skin and the other way around.Trying to eliminate it completely is a fruitless task. At this point you react, like the manager should have talked with the sales clerks boss, find out what is going on.