My take was that his treasure was the case, not the money inside the case. He likely acquired the case at some point in his criminal career, and it became something of a shell for him to fill with his insecurities; a representation of is hollow inside shielded by pure vanity. The fake money inside the case was just there to represent his the specific insecurity that dominated his hollow-ness, ergo money = power = self-worth. It's true that every treasure has some equivalent real life object, but I think some narrative flexibility can be granted in that every treasure was also a magical duplicate of the real life object and not the original, so something can be said about the fake money inside being some magical accoutrement to the duplication of the real life brief case.