As others have said, it was to give himself an alibi after the killing (Fannuci was alive when he left the shop), although the novel explains he didn't need one (The cops only talked to known hoods about Fannuci's murder, and even then they didn't really care too much about it). I also like the theory that it was another test to make sure (one last time, I guess) that Fannuci was NOT a big shot and didn't have any connections to Maranzalla or the Black Hand, although the novel says that Vito's offering less money was permissible (it specifically says that Fannuci might be persuaded to wait longer, or take nothing more).