Thank you for your question. I look forward to working with you to provide you the information you are seeking for educational purposes only.
Legally, when people sign contracts the courts bind them to the contracts they sign. In this case, if you agreed to a particular interest rate and payment and now this has changed, you can seek to argue breach of contract and seek to cancel the sale based on them not providing you the interest rate that was agreed upon and giving you a payment different from what was agreed upon.
You can tell the dealer that you want to cancel the contract based on this breach and if they refuse you are going to report them to the DMV dealer licensing bureau as well as the Attorney General for unfair and deceptive practices and financial abuse of the elderly and you will also report them to the Better Business Bureau. In most cases, the dealers will agree to cancel the sale in this type of situation.
If they refuse, file your complaints with the DMV and AG and BBB and also please go to your local legal aid office or your local senior center (as they many times provide free legal representation to seniors) and engage them to represent you to pursue this company for changing the contract with you and seek to get it cancelled.
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Also remember, sometimes the law does not support what we want it to support, but that is not the fault of the person answering the question, so please be courteous.
This is NOT the practice of law nor is it legal advice to you, it is merely educational information for you to use to seek out a licensed attorney in your state to get actual legal advice from them. Please use sites such as http://www.martindale.com or http://www.lexmundi.com or http://www.hg.org to find a local attorney to get actual legal advice in all matters.