Killing your employer as a mercenary seems like a pretty bad decision. Especially when said employer doesn't need to keep a fortune, since they can make the payment on the spot. And if they break concentration before the first hour after paying you, your fortune goes poof.
On the other hand, you don't think Geased creatures won't want revenge if they manage to get free? Good luck with that. People disappearing seems like a thing that gets adventurers called in. You know, the groups with trackers and spellcasters?