Thank you for your follow up.
Generally, as long as, there is a written agreement signed by both buyer / consumer and the seller and the agreement outlines the terms of the sale and also the value of the trade in, the such agreement would be legally enforceable and either party (consumer or dealer) cannot change their mind and unilaterally declare that there is "no deal".
So generally, if the dealer refusing to honor the terms of the written purchase agreement, then a local consumer protection law attorney should be consulted and this attorney can send legal notice to the dealer to either follow the terms of the sales agreement or to face legal consequences, which can include a lawsuit to enforce the agreement.
You can find an experienced and skilled local consumer protection law attorney by using this established and reputable attorney information / referral websites:
When, choosing a local attorney to represent you, it would be a good idea to choose someone who has a practice exclusively dedicated to consumer protection law, has at least 10 years of experience and handled similar matters and also regularly practices before the court located in the jurisdiction where your matter will be heard.
I wish you the best of luck!