Did you do BOTH parts of the quest??
Scanthril sends you out on the first job to the Stout Mines, when you come back he tries to ditch you in the Boogres Caves but instead of falling for that line you have to ask him again about a "JOB" - then he gives you the second part of the quest. Completing that will result in the Assassin's Dagger!
Wyv