[One Who Bought a Commodity and Increased the Price on Account of Deferred Payment]

The answer: He must repent and seek forgiveness, because he entered into a prohibited transaction that the Law does not permit nor approve; nothing else is required of him.
It would have been obligatory upon the seller to return to him the increase he took on account of the deferred payment. As for the questioner, he has been wronged; he should not have entered into wronging himself. The car he purchased is lawful for him; nothing is required of him regarding it, for he bought it for its price. What is in excess is in the seller’s liability; the buyer’s liability is clear if he repents and turns back to Allah and seeks forgiveness for having entered into what Allah has forbidden.
If it be said: If the transaction is prohibited—displeasing to Allah and impermissible in His religion—then the sale is void; and if void, the buyer does not own the car and must rid himself of it.
We say: The sale, as you have said, is void and should not be entered. However, Allah Most High mentioned in the verses of ribā in Sūrat al-Baqarah the rulings governing transactions of ribā that have already occurred. He, Most High, said: “So whoever has received an admonition from his Lord and desists may have what is past, and his affair rests with Allah.” [Al-Baqarah:275]—thus Allah made what the usury-taker had acquired by usury contracts his property if he repents and turns back to Allah.
And He, Glorified, said: “But if you repent, you may have your principal—you do no wrong, nor are you wronged. And if someone is in hardship, then [let there be] postponement until [a time of] ease.” [Al-Baqarah:280]. Thus He, Exalted, validated the contracts of ribā for the repentant and affirmed them, and commanded him to take only his principal, without the increase he added on account of the deferment.
From here we say: The car is lawful for the purchaser; the usury-taker must return the increase in case of dispute. But if the usury-taker repents, regrets, and seeks forgiveness, then the increase becomes lawful for him by His saying, Most High: “So whoever has received an admonition from his Lord and desists may have what is past, and his affair rests with Allah.” [Al-Baqarah:275]
— Nowadays there are stratagems by which people seek to obtain ribā. A man comes to a bank to be lent the value of a car, or household furniture, or land, etc. The bank proceeds to purchase the car the man wants, or the requested furniture, or the specified plot, and so on; once the bank has bought it, it sells it to the man at a markup over what it paid. This stratagem does not justify the transaction, because the bank in reality purchased the car for the man and paid on his behalf; it did not purchase it for itself—this is the fact—and that does not justify the bank taking a profit.
To clarify: what the bank does in purchasing the car does not meet the essence of commerce, for the essence of commercial exchange is the swap in which the one who enters it is exposed to both profit and loss. The bank entered this arrangement certain of profit and not allowing for loss at all.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2