Even dealers that acknowledge it aren't going to include the holdback in any negotiations. Although they're not required to, dealers usually pass some of this savings on to customers in the form of special year-end clearance sales. Keep in mind that carry-over allowances are in addition to rebates and other manufacturer to dealer incentives. As with most other behind the scenes payments, don't expect your salesperson to acknowledge it, some dealers don't let their salespeople know about them either. Dealer Incentives - Manufactures offer their dealers special incentives on stagnant inventory during different times of the year.
Some dealers only charge for advertising, or split up the overhead in different ways.
It doesn't really matter how they do, you'll end up paying for it either way. However, some unscrupulous dealers see them as icing on the cake, and may try to tell you there isn't one when in fact there is. If a dealer successfully shields the existence of a rebate from a qualifying customer, the dealer gets to keep it. Speak up about any rebates or special financing you know about. Don't fall for this.