daf bit: Bava Kama 93

We have already determined that in the case of robbery the thief owes damages based on the value of the stolen goods. (He pays a multiple of this value.) What if the value changed after the robbery? The mishna on today's daf addresses this. If one stole pieces of wood and made utensils from them, or stole pieces of wool and made garments from them, he owes damages for the value of the pieces of wood or wool. Similarly, if he stole a pregnant cow and it then gave birth to a calf, or he stole a sheep ready for shearing and he then sheared it, he owes the value of a pregnant cow or a sheep ready for shearing. However, if he stole a cow and it then got pregnant and gave birth, or he stole a sheep and it then grew out its coat and he sheared it, then he owes for a non-pregnant cow or a sheep not ready for shearing. This is the general principle, the mishna tells us: all robbers pay in accordance with the value of the stolen goods at the time of the robbery. (93b)

That the thief owes full restoration if he has diminished the value of the stolen items seems obvious to me. That the thief gets to benefit from the proceeds of his theft, for example the calf if the stolen cow later becomes pregnant, comes as more of a surprise to me. What happens if he stole a cow, it became pregnant, and he then paid damages before it gave birth -- if the thief returns the cow, would the owner owe the thief for the increase in value? I suspect that the practical answer is that you treat livestock and goods as commodities -- the thief pays the value of a cow but doesn't necessarily return that specific cow. I'm speculating, and perhaps it's addressed somewhere in the coming pages.