You may not need a contract if you use escrow.com or another escrow company that will agree to split the escrowed purchase money between the broker and the owner/seller after the name is transferred.
This would be a three party deal where the broker would make the deal with the buyer and set up the terms with the escrow company. The owner/seller would then transfer the name to the buyer and the escrow company would in turn pay the agreed upon specified amount to each the broker and the owner/seller.
I don't know what other escrow companies may cooperate in a three way deal such as this but I do know escrow.com has told me via phone they would. Of course the broker would be taking somewhat of a risk that the owner/seller would keep his word and not try to bypass him in some way. However, I believe the owner/seller would have a big incentive to play fair to avoid the loss of future business.