Can a used car dealer verbally add on a late charge for 1 day late months after the contract that was signed said 10 days late there would be a charge?

Can a used car dealer verbally add on a late charge for 1 day late months after the contract that was signed said 10 days late there would be a charge?
No, a Signed contract is Legally binding, you would need permission from a courthouse to modify any such contract.