get a good lawyer and have him draft an agreement between you and the domain owner. even then you are at the mercy of the domain owner that he or she will pay your commission. i assume you are asking because you are worried about that; if that's the case i would probably pass on brokering the deal. the other alternative would be to buy the website yourself, get it transferred into your name with the registrar and then re-sell it as the new owner... hopefully for a good profit. you then have total control over who gets the money.
buena suerte!