Is it permissible to sell a car by installments with the help of a third party who pays part of the price to the seller in exchange for the buyer paying the installments to that person?

This transaction is forbidden by Islamic law because it is an interest-based loan (riba). It involves borrowing one hundred thousand and returning two hundred thousand. It makes no difference that the buyer will be the one to pay it back, because he is paying on your behalf, and the debt is owed by you. Allah Almighty says: "O you who have believed, fear Allah and give up what remains [due to you] of interest, if you should be believers. And if you do not, then be informed of a war [against you] from Allah and His Messenger. But if you repent, you may have your principal – [thus] you do no wrong, nor are you wronged." And the Messenger of Allah, peace and blessings be upon him, cursed the one who consumes riba, the one who feeds it, its scribe, and its two witnesses, and he said: "They are all equal."