Beware of net commission payouts on new construction purchases

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Great question the other day at a continuing education class that no one, including the lawyer teaching the class, seemed to be able to figure out. How can developers sneakily reduce the co-broked commission from what is listed in RLS at close? They seem to tell agents after the fact that the net purchase price they use to calculate commission has reductions built in for any concessions the sponsor had to make, even when it's unsuccessfully making the buyer pay their transfer taxes.

Does anyone know where this is disclosed? How is this allowed by REBNY?