Is it permissible for the first party to take the car, and is he sinful if he does not inform the company of the buyer's intention to pay in installments without settling them? And is the added percentage, as a difference between the cash price and the installment price, permissible or forbidden?

If the money belongs to the first person, he has the right to buy the car in installments from the company or from another source. Jurists have permitted the difference in price between cash payment and installment payment. However, if you know of the owner's intention to manipulate, then it is not permissible for you to buy the car from him, because that constitutes cooperation in sin and aggression, and therefore it is not lawful for you to benefit from any percentage of this transaction.