Nick realizes that Gatsby is offering him an opportunity for services rendered and does not wish to accept it. He spends no time mulling the offer over. Nick feels that the offer is not genuine. He feels that Gatsby made the offer because he was helping him with his reunion with Daisy. It is an insensitive gesture on Gatsby's part. If Gatsby was sincere he would have obliged Nick with specific details about the business opportunity, but he doesn't. The offer was made out of a sense of obligation, not sincerity, and Nick is cognizant of this fact.