Set a thief to catch a thief.

It's an old saying back in my home land, and it contains a grain of sense. Sending one with a similar mind-set to the prey would seem to have some advantage to it, as they would think in a similar way to the one they seek. A dubious character perhaps, but one that can be used to advantage in some ways.
But - handing the key of a bank to a dubious character seems an unwise move. To be sure I did see the key to the Milltown bank handed back to Michael though there was no way I could tell how long since the banker had handed the key over, or how it had been used other than gaining access on the one occasion I witnessed. And there was definitely no way to ascertain if that key has been copied!
I'll watch with interest for any further odd events at the bank in Milltown. And also I will conduct my business with the banker in Dundee or at the 22 guild-holding. Better safe than sorry, according to another saying from home.