From: Rabbi Moshe Sofer
To: Frankfurt am Main
The Chasam Sofer resolves a difficulty in Tosafos regarding a king's obligation to testify in issur matters: although a king must testify about a sotah's adultery due to the issur, the husband cannot administer the oath of shevuas ha'eidus to him, since the king's obligation arises from the prohibition rather than the husband's monetary claim, and the korban applies only when the oath is administered on the basis of a monetary interest.
Source: Shu"t Chasam Sofer CM §31