As you point out, Tom was already inclined to accept Sol's deal--and Sol knew it. He also knew that "Sonny was hot for my deal," so he wanted Tom to rein in Sonny's temper and get him to accept the deal that he was hot for.

Keep in mind what Tom said after the kidnapping: "If your father dies, Sonny--you make the deal." I emphasized "if your father dies" because Tom was giving Sonny the same lawyerly advice he would have given if Sol hadn't snatched him.

BTW: It was the wrong advice. If Vito had died and Sonny made the deal, Clem and Tess, whose loyalty he would have needed, would have lost respect for Sonny because his gaffe at the Sol meeting had emboldened Sol to take his shot at Vito. The troops might even have believed that Sonny welcomed his father's demise so he could be on his own and get into drugs. Sonny would have needed to fight Sol even if Vito died, just to redeem himself.