Is it permissible to deduct from the car's price upon regular installment payments or early settlement?

It is permissible to grant a discount for consistent payment as a form of reward, provided that the price is known and specified before the contract. However, a discount for early payment is not permissible if it is based on a condition between the creditor and the debtor, because this falls under the issue of "forfeit and hasten," which is prohibited by most scholars. Adherence to Sharia regulations for the permissibility of installment sales is obligatory.