From: Rabbi Moshe Sofer
To: Rabbi Yaakov Ber
The Chasam Sofer rules on two independent grounds that the agent who delivered the get may not marry the divorcee. First, his declaration of b'fanai nichtav creates an irremovable suspicion of personal interest, and routing the get through a second agent only worsens rather than eliminates that suspicion. Second, since both parties admit to relations while she was still married, the Torah-level prohibition of asura l'boel applies, rendering the marriage categorically forbidden.
Source: Shu"t Chasam Sofer EH II §53