There is a story in the Gemarah I wont get into the Detail but here is the relevant part of the story. The wife buys a fish and finds a diamond inside are they not responsible to return it to the Fish salesmen as a Mistaken Sale or any other Halachic concern?
There is a big difference between the story of Rabbi Shimon ben Shatach where he returned the jewel and Yosef Mokir Shabbos. The seller over here had no idea he ever had the jewel, he caught a fish and sold a complete fish, lock, stock, and barrel. By Rabbi Shimon ben Shatach he bought a donkey and it had a diamond hanging from itwhich the seller at one point knew was his.