What is the ruling on completing the payment of the price of a car that was purchased in installments with interest, most of its price having already been paid?

If your father bought a car in installments from its owner, and its price in installments was higher than its cash price, there is no harm in that, and he must pay the agreed-upon installments.

However, if he bought the car through a bank that does not own it, but rather pays its price to the seller and takes it from the buyer in installments with interest, then this is pure usury. In this case, your father must try to pay the installments in advance without interest, because interest is usury and injustice. If he is unable to do so, he should pay them according to the agreement, along with sincere repentance, seeking forgiveness, and performing many good deeds.