Originally Posted by Marielle
Interestingly, if you refuse to help him, the knife scene happens automatically when you agree, you have to fail the perception check or you'll notice the dagger, the second time I had to reload five times to fail that unfortunate perception, lol.

You automatically trigger the knife scene if you either tell him he looks capable enough to take care of himself or just "leave", if you want to pick the first dialogue option but fail the perception check you can use a mod from Nexus to enable force fail rolls. I find the mod useful for a few different moments in the game where I want to fail rolls, like when Astarion sees the bear statue in the basement where the Necromancy book is. If he fails a nature check clicking on the bear statue you can hear him say "rawr". xD