Ok, I've talked to the man in the docks, found Isaea's finance book in his house, but that still doesn't impress his boss. How do I eventually frame him? I've searched his house thoroughly and didn't found anything.
If someone could help, I'll keep a good word for him at the Armoury, should he even visit Sigil...

Factotum Raakhaart of the Doomguard

Did you kill the guard in the corner of the Sea's Bounty and take the scroll from his corpse? That's another piece of evidence you need. Forget the armoury, start a tab for me at the inn.