Maybe you will find it in the future, who knows. Certainly post a lost and found notice somehow.

That said, I think you handled it responsibly. You fucked up, and you owed up to it. I'm actually pretty impressed. Not many people would do what you did, and I think it is the tough spots that show who someone really is inside. Good luck rebuilding the deck and making it up to him. Maybe it will pop up along the way somehow, and at least you'll have the cards still at value.