If you follow a specific path, namely without taking the quest to kill him, persuade the king to consider peace with the camp, while also taking his kidnapping quest then go and persuade Edgar (he asks to let him know what was the king's response) and kidnap Sarine the quest for treaty will logically fail. But if you return back to Edgar there is wrong dialogue option that states that Edgar "promised" to teach you to be a better hunter while there was no actual conversation prior to that if you did things in the order described above (because you failed treaty and did not take bounty quests Edgar never did say "here's money, come later for lesson" and there should not be any reward).
Proposed solution: after failing the truce quest by kidnaping Sarine there should be an option to tell Edgar that the King declined the treaty after all (while hiding your involvement) and Edgar offers his regular headhunt quest.
Though this approach will produce a new situation if you attack the king with Edgar party and they discover corpses of Beck rescue group along with caged Sarine and she uncovers your treachery. The combat then starts with both sides hostile to player. On your victory you complete the quest by telling Boyd that you were the only one who stayed alive.
