From: Rabbi Moshe Sofer
To: Rabbi Wolf Leib
A bailee who was entrusted with chametz before Pesach failed to sell it before the prohibition took effect. The Chasam Sofer sides with the Magen Avraham against the Chok Yaakov, ruling that the bailee's permission to sell under hashavat aveidah creates an obligation, making his failure negligence.
Source: Shu"t Chasam Sofer OC §105