What is the ruling on one of my relatives taking my car to sell it and benefit from the money, on condition that he brings me another car of a newer model after 6 months? And is this considered usury if we do not specify the model of the replacement car? And what should I do, as he has already sold the car for a paltry sum that cannot be recovered?

What the questioner and his relative did is forbidden usury, which is a loan in which the lender stipulates an increase or benefit, and this is what scholars have unanimously agreed upon to be forbidden. Both of them must repent and seek forgiveness.

As for what is incumbent upon the relative: he must pay its value at the time he took it, not the price he sold it for, unless the questioner forgives him.

If the transaction was a sale, it is forbidden and invalid due to the ambiguity of the price.

In any case, whether it was a loan or a sale, the transaction is forbidden, and the relative must return the value of the car at the time he took it.