Interesting. To my knowledge anything that has to do with the sale must be reflected on the HUD.
i.e.
I am the listing agent and will buy you a cruise if you purchase the home. I can pay you as a
principal to the transaction and buy you the cruise

; however, it must be reflected on the HUD or it would be Mortgage fraud.
To learn more you can check RESPA (Real Estate Settlement Procedures Act) which governs closing costs and settlement procedures.
RESPA