He wants to refuse, but he can't. He needs the money to continue making something of himself, and he's so busy being thrilled for what Athos wants of him that he's dazed at the thought of getting paid for it, but then it hits him...

"No," he insists, sharp and fast. "No, you can't pay me. Not if we're going to keep doing this," he says, gesturing between them. "I don't care if you're paying me for honest work, it'll feel like you're paying me for the rest."