@Jon Holdman Thank you for the comment. I may not have stated the case clearly enough so I'll try to elaborate further:
The seller knows I was looking for a buyer for him. The deal with a buyer was along the lines of "find me a good deal at no more than $X, and I'll pay you Y% of the deal." The Y% is very thin, as is the assignment fee, but both are a sum I can live with. The legality of our original agreement is not an issue here.
The buyer was willing to pay the full amount to the seller, after which I negotiated the deal further with the seller, who was happy to receive less than the full amount.
I thought if the buyer doesn't mind paying the full amount and the seller doesn't mind getting less than the full amount, why not take the difference for myself as an extra assignment fee. But if the buyer would find out about it, he'd want to "enjoy" the extra savings, thinking he's already paying me the Y% of the deal we agreed upon earlier.
The dilemma was how to get the buyer to still pay (happily) the full amount, have the seller get his reduced price, and let me make the difference, while the buyer doesn't feel as if I "cheated" him by making extra money on top of our agreed upon fee.