Mujahid (may Allah have mercy on him) said concerning the saying of Allah Almighty, "And give up what remains of usury (riba)" [Surah al-Baqarah: 278]: A man would have a debt owed to him by another man, and he would say to him: Give this much more, and pay it after such-and-such a time.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10466