Gatsby wants to repay Nick for arranging the meeting with Daisy. Gatsby thinks Nick might be having some money troubles, so he offers for Nick to go into "business" with him. No one is sure what business this is, but Nick refuses the offer. Gatsby is so grateful to Nick, he wants to do something to repay him. On the day of the meeting with Daisy, it is raining pretty hard, but Gatsby sends a gardener over to Nick's, to have him cut his grass.
Gatsby has spent many years trying to improve his position in society, only so he can have a chance of getting back with Daisy. Now that the time has come for his meeting with Daisy, Gatsby feels indebted to Nick for arranging the meeting. He wants to show Nick his appreciation, but Nick doesn't want to be in business with Gatsby.
Gatsby is not a bad person. He has just let his love of Daisy and his feelings of having to impress certain people. Money seems to be the root of all their troubles in the whole novel. Gatsby has become obsessed with having to have money. He thinks money will bring Daisy back. He thinks Nick will be thrilled to be able to make more money, however Nick seems to be the only one with the level head.
Gatsby, aware that Nick may be in a tough financial situation, offers to "set him up in business" so that he can make some money. He is evasive about what kind of business this might be, however.
First, Gatsby tries really hard not to put Nick out at all. He has Jordan Baker ask Nick for the favor of inviting Daisy to tea, perhaps because he is too anxious to do it himself or perhaps because he doesn't want Nick to feel too awkward about the request. When Nick asks Gatsby when he would like to arrange the tea, Gatsby is very solicitous and wants to make sure that Nick picks a day that is convenient for him. Again, however, this could be because Gatsby doesn't want to seem too desperate.
Gatsby confirms with Nick that Nick doesn't "'make much money,'" and seems reassured when Nick tells him that he doesn't. Gatsby says that he has "'a little business on the side, a sort of side line'" and that Nick could make a nice little sum of money by working with/for Gatsby but that it's a "'confidential sort of thing.'" Nick already knows that Gatsby keeps company with big time gamblers and criminals like Meyer Wolfsheim, the man who fixed the 1919 World's Series. Because Gatsby has always been so secretive about what he does, and because Nick believes that the history Gatsby told him about himself was a fiction, he cuts Gatsby off and declines the offer. Gatsby assures Nick that he wouldn't have to work with Wolfsheim, but he still refuses the offer, which seems to be deeply disappointing to Gatsby who "went unwillingly home."
Gatsby offers Nick a job in his "business" or quote on quote "set him up in the business." Gatsby does this because he knows Nick is not the most wealthiest so he thinks buying his cooperation would be the best thing. However Nick some what sees through it and does not take Gatsby on the offer.