Is it permissible to reduce the value of installments for apartments sold by installments if the delay in delivery caused the buyer to be forced to rent or buy alternative housing? And if this reduction involves sin, what should the buyer do?

The aforementioned contract between the questioner and the owner falls under the permissible contract of Istisna' (manufacturing order), provided two conditions are met: clarifying the type, kind, quantity, and desired specifications of the manufactured item, and specifying the delivery deadline. It is permissible to defer the entire price or pay it in installments, and it is permissible to include a penalty clause. Since the questioner stipulated to the manufacturer a deduction of an apartment's rent from the due installment in case of delayed delivery, the questioner is entitled to that by virtue of the condition, as the manufacturer is bound by it. A penalty clause is permissible in all financial contracts except for contracts where the original obligation is a debt. Therefore, there is no objection to what the questioner mentioned.