The answer to this can be found in Chapter 5. There, Gatsby essentially offers Nick money in exchange for going along with his plan for getting Daisy to meet with him. When Nick tells Gatsby that he has asked Daisy to tea, Gatsby rather awkwardly starts to try to offer him money in a roundabout way. He asks Nick if Nick wouldn’t like to do some work for him. Nick gets somewhat offended by this because he does not like the idea of being paid off for doing Gatsby a favor. Therefore, he does not accept Gatsby’s offer.