Lana, that script line was written for effect--to set up Vito's shooting by highlighting Sonny's greed for the deal in seeming defiance of his father's "no" - the "ch**k in Vito's armor," as the novel puts it. But, if you take Sonny's question at face value, he's being skeptical, like you, as to how the Tattaglia's could guarantee the million dollars Sol wanted from Vito when, if they had a million dollars, Sol could have gotten it from them?