From: Rabbi Moshe Sofer
To: Unknown recipient
In response to a formal inquiry from the Austrian provincial Statthalterei, the Chasam Sofer declares that no rabbinical court has authority to permit any forbidden marital relationship, whether biblical or rabbinic in origin. The sole exception is polygamy, where the cherem of Rabbeinu Gershom includes a built-in dispensation mechanism — the heter meah rabbonim — requiring the husband to deposit the kesubah, guarantee the first wife's maintenance, and swear to divorce her once the impediment is removed.
Source: Shu"t Chasam Sofer EH I §37