Date: 2 Tevet 5576
From: Rabbi Moshe Sofer
To: Rabbi Meir Ash
The responsum discusses the Ra'avad's opinion on wheat taken to a non-Jewish miller, which becomes Demai (doubtfully tithed produce) due to concern that it might have been exchanged for untithed grain, and analyzes the concept of 'safek s'feka' (double doubt) in this context.
Source: Shu"t Chasam Sofer Collected §11