Is it permissible to stipulate a penalty clause amounting to 30% of the amount paid upon the termination of a contract for purchasing a housing unit in installments due to payment default, knowing that the contract was signed after it was deemed permissible by a fatwa, and then some of its conditions were declared impermissible?

It is not permissible for a manufacturing contract (Istisna') to include a penalty clause on the buyer (mustasna') if he delays in paying the price, because it is a debt, and any increase on it is usury (riba). However, a penalty clause is permissible on the manufacturer if he delays in delivery. It is generally not valid to stipulate compensation for rescinding a sales contract, because it constitutes consuming wealth unlawfully. If it is possible to annul the contract without significant harm, or to renew it to invalidate the corrupt condition, then do so. Otherwise, continue with the contract, and there is no sin upon you if you were unaware of the condition's impermissibility.