What is the ruling on working as a real estate broker for companies that impose late payment penalties on installments, knowing that the broker does not write the contract or sign as a witness, but may review it for the buyer? And what is the ruling on selling units under the "cash on down payment" system, where the client pays 30% and receives a 17% annual return for 3 years on the down payment?

It is impermissible to impose a fine for late payment of purchase installments because it constitutes usury (riba). Companies are not permitted to include such a clause, nor is it permissible to cooperate with them in this regard. As for the system of return on down payment, if the sale is legitimate and the down payment was made as part of the price, there is no basis for the buyer to receive a return on it. However, if the down payment was intended to demonstrate seriousness or to benefit from a return, then this scenario is forbidden, because the amount becomes a loan, and it is impermissible to stipulate a loan within a sale, based on the hadith: "A loan combined with a sale is not permissible." If the seller invests the money on behalf of the buyer, then a return that is a percentage of the capital, coupled with a guarantee of the capital, is considered an usurious loan.