Is it permissible, Islamically, for a construction company to confiscate all advance payments and due installments, in addition to 20% of the total contract value as compensation, if the buyer withdraws from purchasing a house by installments?

The contract as described is not permissible due to the presence of jahalah (uncertainty), gharar (risk/deception), and the unlawful acquisition of wealth (akl al-mal bil-batil). This is because the value of the down payment and the installments due are unknown. The penalty clause (shart jazai) is intended to compensate for actual damages, not as a means to seize the down payment. A penalty clause is valid and considered permissible unless there is a legitimate Shariah excuse, or if it is excessive by common custom and intended as a financial threat. In such cases, fairness and justice must be observed, and the assessment of this matter, in case of dispute, is referred to the Shariah judge.