I liken the comparison to earnest money on home that was mentioned earlier. Typically earnest money is 1% of the price. 2 or 3% if there are some special terms involved, pulling a home off the market, buyers from out of town, etc. But it sounds like you are closer to 10%. If it is written up well and he agreed to it, then well to what you feel is right
In a home sale though the sales agreement clearly states what happens to the earnest money if one party defaults and the sale does not go through. I am guessing that if there is not similar wording in the agreement the OP wrote up that he could not legally keep the money. Sure he could keep it anyways but then he might be open a lawsuit to get the $1000 back.
I am not an attorney though, so IMO the OP shoudl check with one first to be sure.