I always questioned why Sollozzo thought that Sonny's malaproprism indicated that he would go for the deal. It could have just as easily been interpreted as Sonny's immature outburst about a relatively incidental part of Sollozzo's proposal. In any case, Vito's refusal had to be anticipated by Sollozzo who would have already planned his murder if he refused.
Basically Vito told Sollozzo "NO." Sollozzo then wanted to continue to negotiate and says something to the effect if it is finance Vito is worried about, the Tattaglias will agree to guarantee his investment. Sonny then jumps in and takes the bait, as if something were still negotiable. That tipped his hand.
