Bechorot 3 - Selling an Animal to a Non-Jew
We learned that although selling an animal to a non-Jew removes it from the laws of firstborn, in general it is forbidden to do so. Why? Some say that since a Jew does not make his animal work on Shabbat, and the non-Jew does, we should not remove the animal from the Shabbat observance. Others say that once an animal is completely owned by a non-Jew, its Shabbat observance should not concern us. Rather, if one is allowed to sell livestock, eventually he may come to rent it out, and then the Jew will have caused his animal to work on Shabbat. Finally, some say that "forbidden to do so" refers here to removing the animal from the sanctity of the firstborn. (Nowadays the custom is to permit this).
Rabbi Yehudah allows selling an animal with a broken leg, because it can’t work anyway. What would he say about selling a firstborn fetus? Is it allowed, because it can’t work now, or is it prohibited, because it will be able to work later? Since in another place he gives the reason "because it cannot heal," and this reason does not apply to a fetus, we deduce that he prohibits it.
Art: Sydney S. Morrish - Sabbath Evening In A Shepherd's Cottage