Ron, the best way to do this is to "sell" the contract to the final buyer. That way, you're assured of your fee regardless of whether the buyer finally closes or not. Your job has been to locate and negotiate the best deal, and the buyer is the beneficiary of your hard work; therefore, what you are selling is the negotiated "deal", not the actual property. This has worked for me in the past. It doesn't always work if the buyer in not an experienced or sophisticated buyer, as they think they are paying an additional commission.
Mar 27, 2012