Is it considered a mistake not to pay the remaining amount to the seller, knowing that he cheated in the sale and that the car is not worth the price paid?

What is appropriate in such cases is reconciliation with the adversary or resorting to the judiciary to resolve the matter.

If what occurred was an unfair price that was confirmed for the unsuspecting buyer, then he has the right to either annul or uphold the contract.

However, if it was fraud, which is concealing a defect in the sold item, then the seller does not deserve from the price an amount equivalent to what he defrauded, but neither party has the right to determine that.