From: Rabbi Moshe Sofer
To: Rabbi Moshe Brezeve
The Chasam Sofer analyzes whether a firstborn son may be redeemed through a kohen's agent or a minor kohen when no adult kohen is locally available. While strict Talmudic reasoning would permit both options — distinguishing the act of giving (nesinah) from legal acquisition (kinyan) — he rules that in practice one must not rely on this, since the Geonic liturgical formula for pidyon haben requires the kohen's personal participation, and one should instead bring an adult kohen from elsewhere rather than delay the mitzvah.
Source: Shu"t Chasam Sofer YD §292