What a great memory you have, to remember Thomas Mitchell as the actor playing the doofus uncle of George Bailey. That Potter proved himself to be utterly wicked by keeping the money. Would it not have been a crime for him to do so?
Well, Potter was nothing if not an extreme stereotype of the evil bastard manipulator, a lot like George Soros of today, but on a local level. And of course it was a crime since he KNEW it was left there accidentally by Mitchell and once he seized the 8K it provided exactly the leverage he needed to throw the Bailey S&L into insolvency.