وَعَنِ ابْنِ عُمَرَ: أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسلم نهى عَن بيع الكالئ بالكالئ. رَوَاهُ الدَّارَقُطْنِيّ

Ibn ‘Umar said the Prophet forbade selling a debt to be paid at a future date for another (This is allowing a man who cannot pay a debt when it is due to have an extension of the period in return for an additional sum payable; or when a man agrees to sell an article which someone owes him for money which someone else owes to the buyer. Daraqutni transmitted it.