Is it permissible to exchange a car bought in installments—the full price of which has not yet been paid—for another car, with the first car being valued as part of the deal and the price difference for the new car being paid in installments?

It is permissible to sell an old car for a new one with the payment of the difference, as long as the cars are not from the usurious categories. There is no harm in disparity or deferment in their sale. However, for someone who bought a car in installments, it is not permissible for him to sell it for a lower cash price to the original seller, unless its condition has changed in a way that decreases its value.