It's not so much the fact that Sonny says it in disbelief; it's the fact that he's not agreeing with Don Corleone that makes Sollozzo initiate the hit.

Sollozzo may not have actually thought Sonny WANTED to accept his deal, but he wasn't 100% against it like Vito was, so there was a chance that with Vito out of the picture, Sonny would go along with the deal to keep some of the Corleone family's power.