Is the money received for keeping comprehensive car insurance permissible, knowing that this feature was mentioned in the sales advertisement?

It appears that mentioning the feature of comprehensive insurance in the car's specifications impacts the price, as it implies that the sale includes both the car and the insurance. If this is the case, the seller has no right to take any money from the buyer in exchange for the insurance remaining, so long as this was not stipulated in the contract. The seller must return what he took. His statement that the buyer gave it to him voluntarily is incorrect, because he threatened him with canceling the insurance if he did not pay him the money. So the buyer paid to keep the insurance, which is his right by virtue of the contract.