Tom Hagen is not on the muscle end of the family so he was obeyin Sollazo until he got out of there. That is why there is the phrase "nothing personal, just business". Even though they shot the Don, if they didnt make the deal the other families would have tried to wipe out the Corleone family. Hagen knew they couldnt afford a stalemate and the only way to stay in the game would be to go with Sollazo's deal. Even after Don Vito had been shot he agreed to make the deal because he knew if he didnt more people would be hurt in the family. Tom Hagen was the most loyal person in the family. (Also for side reference Sollazo told Hagen "Well that's bad luck for me,
and bad luck for you if you don't make that deal!", that threat meant they would be promblems for Tom and the Corleone family if Hagen didnt convince Sonny to make the deal.)ANother way of thinking about it is if Tom would have told SOnny "Sollazo wanted me to ask you to make the deal" would SOnny do it. Sorry to say he was hard headed just hearing Sollazo's name would be him deny the deal/