From: Rabbi Moshe Sofer
To: Rabbi Zalman Ullmann
The Chasam Sofer analyzes whether a Jewish householder's sale of a firstborn cow to his non-Jewish servant was legally effective, concluding it was likely valid through kinyan kesef since most Rishonim hold a non-Jew acquires through money. Regarding inflicting a blemish on the bechor, he rules it is only a rabbinic prohibition in the present era both because we are outside Eretz Yisrael and because bechoros cannot be eaten even in Eretz Yisrael today; he does not permit a Jew to inflict the blemish but suggests relying on a minor non-Jew to do so.
Source: Shu"t Chasam Sofer YD §318