Date: 26 Shevat 5584
From: Rabbi Moshe Sofer
To: Rabbi Asher Zelig
Rabbi Zelig inquires about the Chasam Sofer's reservations regarding reciting 'HaMavdil bein kodesh l'chol' on Motzaei Shabbos, and which textual variant is correct. The Chasam Sofer explains that while he follows the prevailing custom, the piyut's themes of forgiveness and Neilah indicate it was composed for Motzaei Yom Kippur, and rules that 'kaspeinu' before 'zar'einu' is the authentic text.
Source: Shu"t Chasam Sofer OC §67