I know if this happened to me I'd have to return the money if I was asked politely. It'd be way too much of a burden on my conscience just because I know what it's like to lose that kind of money for a stupid reason (servers were down during gardening feeding time for 15 ore gardening mules).
If I did it by accident I'd ask to have the money returned politely and if it was returned I'd give back 10% for the act of kindness. If I didn't get the money back I'm not sure how long I'd have to stay out of the game for to calm my RAAGE!!!