halacha - Riches found in a purchased item. Return or keep it?

A young rabbi reportedly found nearly $100,000 in a desk he'd purchased second-hand on Craigslist, and subsequently returned the money.

Is this story about a Kiddush HaShem, or a Hassid Shoteh (stupidly/unnecessarily pious person)?

Answer

Thanks to Reddit user Bar Kappara (by way of @Isaac Moses) for this answer:

Devarim Rabba 3:3: It is told of Rabbi Shimon ben Shetach that he once purchased an ass from an Ishmaelite. When his disciples came, they found a jewel suspended from its neck and said, "Master, 'The blessing of Hashem, it makes rich.'" [Proverbs 10:22] Rabbi Simon ben Shetach said "I purchased an ass --- I did not purchase a precious stone." He therefore went and returned the jewel to the Ishmaelite, who thereafter used to say, "Blessed be the Lord G-d of Shimon ben Shetach."