What is the ruling on the maintenance costs of a car that one of the relatives repaired and then backed out of buying after using it for a month and a half?

A sale is a binding contract, and neither party may unilaterally revoke it except with the consent of the other, unless an option (khiyar) exists. Rescission (iqalah) is commendable, but it is invalid if the value of the sold item has increased due to substantial improvements, as this increase prevents the validity of the rescission. Therefore, the questioner should return the car and reclaim his money. They can then enter into a new sales contract at a price they both agree upon, taking into account the value of the previous usage and the improvements that increased the car's value.