I've just checked it out, and that was the scene that I was remembering. I hope it's the one that di79mar wanted.
It was Rayford Flicker of United Amalgamated Dixie Charities, Inc. who was collecting for the Orphans' Fund. Boss starts the donations with $500, but while the box is on the Duke Boys' table, a distraction created by Rosco allows Rayford to drop the money into the false bottom of the box so that they can say the boys stole it. The scene is about 20 minutes into the episode, which is #16 in season 4.