Racercrazy, Give marshall back his money. If you don't than all your earnings will get Haram. Honesty in dealings and truthfulness are the main traits of a Muslim. If you do not care about that you are out of a respected Islamic Circle...it is as simple as that.
Plus you will loose out this money in some other shape and form sooner or a later and in very near future. It would be a Naik act of you if you do so and will earn Sawab and more respectable in the eyes of the creator...let alone the humble humans.
Please let us know asap that you have returned the money. From your history you do not seem to be in desperate need for a few thousand...if you do not have this than we will do sadqa for you. I volunteer 1000Rs if you ask for help?