When the sheriff tells Jefferson he's going to get a "fair" trial:
> "You're going to get a fair trial, Jefferson. That's what you're entitled to. We're not going to let anyone railroad you." (Chapter 4)
The reader knows the trial will be anything but fair. The sheriff's statement is clearly ironic, highlighting the deeply ingrained racism and prejudice that permeates the Louisiana justice system of the time. This ironic statement, though not explicitly sarcastic, creates a sense of bitterness and hopelessness for Jefferson and the reader.
While sarcasm might not be a direct literary tool used by Gaines, the novel relies heavily on irony and understatement to expose the stark reality of the characters' lives and the injustices they face.