Is it obligatory to repay the entire borrowed amount before the debtor's repentance is accepted, and is it permissible to calculate the price of the car that was relinquished to settle part of the debt at its installment price or its cash price?

Your remorse and regret are proof of the sincerity of your repentance, but its completion lies in returning the right to its owner. The right has become a debt upon you, and your sincere intention to repay it may be a reason for Allah's help to you. If you wish to give him the car in exchange for his right, the consideration is the price upon which you both agree, and you cannot compel him to accept it. The current inability to repay the right does not prevent your supplications or worship from being answered. Your repentance is valid and accepted, Allah willing, but it is not complete until you repay the right or its owner absolves you of it. Have a good opinion of Allah regarding the acceptance of your repentance, praise Him for guiding you, and increase your supplications for Him to keep you steadfast.