Anton's last purchase was a gaudy pin, round and covered in fake diamonds. It cost Anton $1.00, and Patty thought it looked like nothing Anton would ever buy. She already felt that she knew him and could guess his tastes, so it was surprising that he would buy such a large, ugly pin, especially showing the joy that he did.
However, that was exactly what Anton wanted out of it. He wanted something that looked big, gaudy, and expensive without being so. He wanted something that would not look like something he would have. That way, later on, he could use it to buy his freedom from a greedy, financially troubled guard.
The pin has symbolism in the story as a representation of freedom, provided one does not attempt to be oneself. Anton could be free as long as he could convince people he was someone else, and not a disguised Nazi. Patty could have some sense of freedom from her family only if she pretended to be a vapid, girly sort that she certainly was not.