Solozzo felt that he "was invulnerable" and "noone would shoot down a police captain, never". Michael was regarded as a civilian and not up to the murdering end of the family.

The entire idea of the meeting is to buy time for Solozzo so he could hit "the old man" again. He needed to give a deal which would satisfy the Corleones, until he could do the deed.

In the book, Solozzo's meeting is one of assurance that he would not kill Don Corleone, but needed to do some business so that he could be making dollars. McClusky was really needed to keep that part going (what better way to make sure drug dealers pay, and limit the amount of arrests, and confiscate any evidence of illegal activity?)

This proves that Solozzo did not have the money to pay a "negotiator" (remember, he had to hire one for Michael) and you know that Sonny was not going to pay one for Solozzo.