What is the ruling on a merchant selling debts owed by debtors whose payment due dates have not yet arrived, to another person who waives 50% of the debt in exchange for collecting the remainder? And is this transaction considered a sale of deferred payment for deferred payment (بيع الكالئ بالكالئ)?

This transaction falls under the category of selling debt for debt, which is prohibited for several reasons:

1. Selling Debt for Cash: This involves both riba al-fadl (usury of surplus) and riba an-nasi'ah (usury of delay), as the buyer pays less to receive more of the same type after a period of time. 2. Selling Delayed for Delayed (Al-Kāli' bi Al-Kāli'): This is because the buyer pays a large portion in cash, and the remainder becomes a debt owed by him for the goods. 3. Gharar (Uncertainty): This is because the debt (the new buyer's debt) cannot be guaranteed to be delivered; it might be repaid or it might not. The prohibition of selling what is in people's liabilities is due to the fear of gharar and non-delivery.

As for the hadith "selling delayed for delayed," it is "weak." It has been deemed weak by Ibn 'Adiyy, Ibn Hajar, Al-Albani, and others.