Lol. This is actually a sample question in my law class right now. No, it’s not yours. The law that governs “finders keepers” says you must make every effort to return the goods to the owner or they can sue you for it. If they were dead, or you had no idea their contact info - you’d be in the clear. But they didn’t intend to leave it, it was not part of the agreement of the sale of the home, and they are asking you for it. Your trying to get an extra $300,000 above and beyond the price you paid for the home. You can try to sue, but you will loose. YTA. Give it back. You didn’t earn it, and their mistake doesn’t automatically make their $$ yours.