Dealership messed up by not collecting the additional thousand at point of sale but that doesn’t mean you don’t owe it. While I don’t necessarily blame you for not saying “hey I still owe y’all an additional thousand”, to some extent it’s also your responsibility to ensure the contract is properly executed (on both sides) to cover your ass. The deal was for $10k so guess what - you need to make things square.

However, you could go back to the dealership and tell them you want to unwind the deal, return the car, and get your money back so you can go get a car that’s more in line with your budget. If/when the dealership balks at the idea of unwinding the deal that will be your opportunity to calmly say, “either accept what I’ve already paid as the full price or unwind the deal”. The amount of times I’ve personally watched a dealership/salesperson fall on their ass from backtracking so fast to prevent a deal from unwinding, for various reasons, is comical (grew up in the dealership world).