From: Rabbi Moshe Sofer
To: Rabbi Chaim
A Jew sold the fetus of his cow to a non-Jew with money alone, without meshichah, and the cow gave birth to a male firstborn. The Chasam Sofer rules that the animal is exempt from bechor status under virtually all halachic opinions, demonstrating that money was sufficient to effect the sale to the non-Jew. As a practical fallback for any remaining doubt, he advises having the non-Jew take possession and inflict a blemish on the calf, after which the Jew may repurchase it as a doubtful bechor with a blemish that a Kohen cannot claim.
Source: Shu"t Chasam Sofer YD §312