That was the exact problem I had. I went to the stout mines first, and killed him on my own. That got Scanthril in a rut, and wouldn't give me the dagger from then on out. I had to restart my adventures. Took me about 6 hours to get back to where I was.

I believe I have discover why some of us are unable to get Scanthril to give us the Assassin Dagger even after Raskalion has been killed.
I went back to my original dialogue with Scanthril and found that the task I was given was to find Grundel, not to kill Raskalion. I now realize that I discovered the existence of Raskalion, not from Scanthril, but from the Bartender back in town.
I am assuming that you must get the quest from Scanthril before you kill Raskalion. If you don't it's too late. Even if you manually type in Raskalion's name after the killing, Scanthril does not respond.
C'est la vie!
p.s. I've been gaming for a very long time (Apple 11, Wizardry 1) and have seen it all. I must say that this is the most erudit(if this is an appropriate term for a gaming board), helpful, and friendly Board I have ever come across. Kudos to all!

Good point Larry! I had forgotten about the bartender.

If all you wanna-be assassins want to play it on the safe side, make sure you pick up the hot tip from the bartender in Valeia before talking to Scanthril or going to the Stout Mines!

Hmmm I got the assasin quest. Visited the stout mines and found a body near were you meet Rask.in the stout mines. If I remember right Scanth ask you to find Grundel and if he has not killed Rask then he expects you to finish the job. No ID on the body (similar to that outside with the writ) but I killed Rask. Upon returning to scanth he says Grundels dead. You have to ask for the reward which he grudgingly gives and I got the dagger no problem.