You need to shop banks for financing. Average used car rates look like they’re around 5.5% for good credit buying used. https://www.businessinsider.com/personal-finance/average-auto-loan-interest-rate

Find some local banks and some national banks, get a few quotes. Sounds like this guy doesn’t know what he’s doing.

If you really want to annoy him, make sure the loan he’s getting you is prepayable without penalty and then get refinance quotes from banks. Get the lower price they give to people who finance with their crappy credit company, and refinance it the next week when the paperwork is all settled. They’ll probably lose out on the financing charges /bonuses.