How did Cassie insult MrBarnett?

Cassie marches over to Mr. Barnett to remind him that he has forgotten about them. He responds angrily, telling her to get her "little black self" back to waiting and calling her a "little nigger." Cassie protests, until Stacey begins to drag her out of the store, and Mr. Barnett tells him to "make sure she don't come back till yo' mammy teach her what she is."