Gatsby offers to help out Nick financially through his notorious connections. It's an expression of gratitude for Nick's helping to arrange Gatsby's clandestine meetings with Daisy and making available his cottage for the purpose. But Nick is not interested. He helps Gatsby out of friendship; any kind of monetary reward would be entirely inappropriate. As far as Nick is concerned, he is simply doing a favor for a friend and that is all.
Nick also has moral qualms about any kind of financial reward. He doesn't like the idea of being paid for something that comes across as vaguely immoral. This episode illustrates that Nick, unlike Gatsby, still has an attachment to old-fashioned Midwestern standards of decency and propriety. Gatsby has been corrupted by wealth; he's used to dispensing money to people who do him favors. In the shady world in which Gatsby operates, this is perfectly normal—but not in Nick's world. There's a major culture clash here, and Nick reminds us of the world that Gatsby left behind, a simple world where traditional values such as pure friendship prevail.