I would return extra change if I discover it before I completely leave, or it's a larger amount.
As for the extra burger, I keep that. They can't resell it, they would just have to throw it away anyhow, you know. Besides, they have shorted me so many times, I would consider it my due. lol

However, that's a completely different situation. The seller knew it was a female animal which is potentially capable of producing offspring every year. They KNEW that they paired her up. If they were not willing to wait to make sure that she wasn't gravid--absolutely sure--before they sold her, then of course they knew that they might be selling a gravid animal. I don't see this as any different from your buying a female that ISN'T gravid, and then breeding it and winding up with offspring just a few months later. In fact, maybe you had plans for that female, sold as being not gravid, and now they were interfered with because she turned out to be gravid. Now you can't pair her with YOUR male.

They sold a female that they had paired up with their male. They're giving you the result along with her, and they KNOW that. I wouldn't feel guilty over receiving the animal and getting offspring from it. Obviously they wanted to sell her...if they wanted her offspring, they wouldn't have sold her.

It's not ALL roses for the buyer. What if the buyer was planning to pair the female with a high-end animal, and instead it lays eggs sired by the seller's low-end animal? They've lost out on the potential from her for that season, and the clutch they do get is only a minor compensation for it.

So, to simply things--really, the animal and anything it produces belongs to the buyer, free and clear, without obligation, and certainly without guilt.