Why doesn't Uncle Hammer want Stacey to get the coat back from T.J. in Roll of Thunder, Hear My Cry?

Uncle Hammer doesn't want Stacey to get the coat back from T.J. because he wants to teach Stacey a lesson.  Stacey showed very poor judgment in caving into peer pressure in giving up the coat, so Uncle Hammer decides that he doesn't deserve to have it.  Uncle Hammer hopes that by letting Stacey suffer the natural consequences of his foolishness, he will learn from the experience and act more wisely in the future.

Uncle Hammer had given the coat to Stacey.  Although it was a little large for him, it was a good coat, "the best coat Stacey'(d) ever had and probably ever would have as long as he live(d) in (the Logan) house".  At first he had been delighted to have it, but then T.J., recognizing the coat's value, began teasing Stacey about the coat, making fun of its largeness and telling him it made him look like a preacher.  The other boys soon began to follow suit, at which point, the manipulative T.J. offered to take it off Stacey's hands so the kids would stop mocking him.  Stacey, sensitive to his "friends'" teasing, foolishly succumbed to T.J.'s manipulations and gave up the coat.

Uncle Hammer is angry that Stacey gave away the coat, but he is more concerned that his nephew showed so little backbone in standing up for himself.  Like the other adults in the Logan family, Uncle Hammer recognizes that, because of the color of their skin, the odds are stacked against their children in the social climate of the deep South during the Depression years, and they struggle to give Stacey and his siblings the wisdom and strength to survive in a world where prejudice and injustice are rampant.  Uncle Hammer tells Stacey,

"If you ain't got the brains of a flea to see that this T.J. fellow made a fool of you, then you'll never get anywhere in the world.  It's tough out there, boy, and as long as there are people, there's gonna be somebody trying to take what you got and trying to drag you down.  It's up to you whether you let them or not...You care what a lot of useless people say 'bout you you'll never get anywhere, 'cause there's a lotta folks don't want you to make it.  You understand what I'm telling you?" (Chapter 7).