A manufacturer or distributor motor vehicle price increase shall not apply to vehicles which the dealer had ordered for private retail consumers prior to the dealer’s receipt of the written official price increase notification. A sales contract signed by a private retail consumer shall constitute evidence of each such order. In the event of price reductions, the amount of any such reduction received by a dealer shall be passed on to the private retail consumer by the dealer if the retail price was negotiated on the basis of the previous higher price to the dealer. Price reduction shall apply to all motor vehicles in the dealer’s inventory that are subject to the reduction. Price differences applicable to new model motor vehicles at the time of the introduction of new models shall not be considered a price increase or price decrease.