Atticus states that some day the bill for the white peoples unjust treatment of blacks will come due. What does this mean?

Atticus is speaking about the greater injustice that has plagued the South. Slavery and subsequent bigotry of the black population comes with a price. There would eventually be repercussions for the actions of racist whites. I'm not sure if Atticus knew what those repercussions might be but he knew things would change in America.