Oh, that's the fun part. The BULK of his fortune? It's invested in banks and properties all over the world.
The cash in the Bin? That's his SOUVENIR money. It's all the money he earned his own damn self, from his first crappy jobs to his first big score. He knows the story of every damn penny in that Bin. They're like old friends to him.
So, yeah. The Money Bin isn't Scrooge McDuck's piggy bank. It's his coin collection. The Biggest Damn Coin Collection Ever.