Stacey gave the coat away to T.J.
In Chapter 7, we discover why Stacey isn't wearing the coat that Uncle Hammer gave to him: T.J. has it in his possession. Essentially, Stacey gave the coat to T.J. because the latter made fun of him. For his part, T.J. manipulated Stacey into giving away Uncle Hammer's coat by playing on Stacey's need for social approval.
Cruelly, T.J. maintained that Stacey would look like a preacher in the coat. This is the main reason Stacey no longer has the coat Uncle Hammer gave him. When Uncle Hammer discovers what happened, he orders Stacey to refrain from trying to get the coat back. In this, Uncle Hammer means for Stacey to learn a hard lesson about the world.
According to Uncle Hammer, Stacey must learn how to protect the things he cherishes. Additionally, he must also learn to be watchful, in order to prevent unscrupulous people from taking advantage of him. Here, Uncle Hammer is also indirectly saying that T.J. lied to Stacey.
T.J. wanted the coat for himself, so he used verbal manipulation to get his way. He never intended to give Stacey the coat back.