I'm not sure what happens if you free Saul first. You can simply refuse to accept the quest from Tilian, though. IIRC you still get a reward freeing Saul, even if you fail the 'thorn in the side' quest (I killed him in a save I was planning to reload anyway, and then freed him; if there was a difference in reward it wasn't enough to get me to reload again and kill him again).
