From: Rabbi Moshe Sofer
To: Rabbi Eizik Leib
A buyer sent a non-Jewish wagon driver to purchase whiskey before Pesach but forgot to include it in his chametz sale. The Chasam Sofer rules the whiskey is permitted to the seller, since the buyer never legally acquired it, but it is forbidden to the buyer as a rabbinic penalty for his negligence.
Source: Shu"t Chasam Sofer OC §133