All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon Prophet Mohammad and upon all his family and companions.

From an Islamic point of view, the seller is not obligated to inquire about the source of the buyer's money because the sin of acquiring unlawful wealth is associated with the person who earned it, not the money itself, unless the specific money being used is stolen.

A Muslim doesn't incur sin for selling a product to someone who has previously taken a usurious loan. This is because the usurious loan was a past transaction with no direct connection to the sale contract, and the loan's approval was not contingent on the completion of the sale. Therefore, there is no linkage between the two transactions, nor does this sale contribute to facilitating the usurious contract.

However, it is prohibited for a Muslim to sell their product to someone intending to purchase it using a usurious loan that can only be granted by providing the necessary documentation for completing the sale contract. This constitutes aiding in a prohibited act, as Allah Says (What means): "And cooperate in righteousness and piety, but do not cooperate in sin and aggression. And fear Allah; indeed, Allah is severe in penalty."  [Al-Ma'idah/2].

The distinction between the two cases is clear: the first involves no assistance in committing a prohibited act, whereas the second does involve aiding in such an act.

In conclusion, if the buyer of the car has already taken the usurious loan and brings the money to purchase the car, there is no sin in selling it to him. The sin of engaging in usury lies upon the one who dealt with the bank. However, if the buyer has not yet obtained the loan and requests documents to facilitate completing the loan transaction, it is prohibited to assist him in this. And Allah The Almighty Knows Best