When Vanessa finds out she didn't get chosen as drum major, she hits Theo on his left arm. Throughout the conversation, Theo holds or rubs his left arm. After Vanessa leaves, he's rubbing his right arm.
Slim gives Russell's $100 bill to Vanessa so he can continue claiming that he never accepted repayment of Russell's debt. Technically, though, by choosing to give the money to Vanessa, Slim is acknowledging that it's now his own money to do with as he chooses, and so the debt is indeed repaid.