Reference for Bekhorot 4:29

הא יכולה להתרפאות אסור והאי נמי כיכול להתרפאות דמי

Now if [the words 'its value'] refer to the animal, what has the Priest to do with it? - We are dealing here with a case where e.g. , an Israelite gave him a pregnant animal to fatten, and since we punish him for [selling] the animal [to a gentile,] we also punish him for [selling] an embryo.

Jastrow