I would return the money. I know how much losing $10,000 can destroy you.
When I was in college, I was at an amusement park and got on the bumper cars. When I was climbing in the car there was a fat wallet on the seat. I looked quick and saw it had about $600 in it. Back in the 70s, that $600 would have paid my college tuition. I never thought twice about it and turned the money in as soon as the ride was over
About two years ago, I put my wallet on top of my car, while putting groceries, into it. Then, without thinking, I drove off and after about 2 minutes, realized I had not taken the wallet off the car. Turned around, ran frantically into the store, where someone HAD turned it in, but removed the two 20 dollar bills that were in it.
I had mixed emotions about the theft, but after sorting it out in my head, decided to be grateful that I had all my other precious things I keep in my wallet, and hoped the money, helped someone out. Win/Win.