He aims for me to lie […]. And I will have to do hit [it]. (10)
Even though lying isn't usually a courageous act, Sarty is mustering his courage to do what his father wants him to do. At this stage of the game, he is a brave person, but his bravery is being used to serve his father.
There was something about his wolflike independence and even courage, when the advantage was at least neutral which impressed strangers […]. (26)
In other words, people see Abner's complete lack of fear in any situation as impressive. So long as they don't think he wants to hurt them. He's willing to go anywhere and do anything, as long as he gets to decide where and what.
"He ain't done it! He ain't burnt…" (70).
Sarty obviously thinks they are in court because his father burned the de Spain barn in retaliation of de Spain's behavior regarding the rug situation. Because Sarty believes his father acted rightly and de Spain wrongly, he has the courage to lie for his father.
This is how the breathless Sarty warns de Spain that Abner is about to burn his barn. Because he finally feels confident that he's doing the right thing, his courage is up. This courage propels him forward.
He did not look back. (109)
The story's final lines make it clear that Sarty is leaving home forever. This is the courage icing on the courage cake he made by defying his father and trying to stop the needless destruction of the de Spain barn. Courageous as this moment is, it's also sad. Sarty is now completely alone.