Does a real estate contracting company have the right to demand a difference in costs from customers who have not paid the full price, after an increase in the prices of iron and cement?

If the relationship between the questioner and his clients is one of buying and selling ready apartments, then it is not permissible to demand an increase in the agreed-upon price.

However, if the relationship is a manufacturing contract (Istisna') for apartments under construction, and the contractor was harmed by an increase in costs, then it is permissible to demand the difference. In this case, the judge or arbitrator shall resolve the dispute, in accordance with the resolution of the Islamic Fiqh Academy regarding unforeseen circumstances (force majeure).