From: Rabbi Mordechai Leib Winkler
To: Rabbi Shabtai Friedman
The responsum discusses the case of a firstborn animal sold to a gentile servant at the time of birth, and whether the sale is valid if there is a doubt about the fetus having partially emerged, citing a dispute between Rav Huna and Rabbah in Chullin 69b.
Source: Yoreh Deah I §211