How we cite our quotes:
"Damnation! Send him out of here!" (11)
Harris makes the choice not to make Sarty testify. He realizes that if Sarty doesn't testify, he has nothing. That's why Harris is angry. His sympathy for the boy overrides his desire to see Abner brought to legal justice.
"Answer me," his father said.
"Yes," he whispered. (29, 30)
Sarty knows that he either agrees with his father about the motivations of Harris and the Justice or he gets hit again. He chooses to lie to his father here to spare himself pain and trouble. Being forced to agree with something he doesn't believe in is part of what makes the experience so frustrating for him.
[…] the boy […] saw the stiff foot come squarely down in a pile of fresh droppings where a horse had stood in the drive and which his father could have avoided by a simple change of stride. (41)
Even though this moment is revealed through Sarty's eyes, he doesn't seem to quite understand that his father chose to track horse poop into the de Spain house. He can't quite imagine his father deliberately doing the deed. Otherwise, he probably wouldn't feel sorry for his father when de Spain belittles him.