Imam Malik (may Allah have mercy on him) said that a man came to Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) and said: O Abu Abd al-Rahman! I lent a man a loan and stipulated that he repay me more than what I lent him. Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: That is usury (riba). He said: O Abu Abd al-Rahman! Then what do you instruct me to do? Sayyiduna Abdullah (may Allah be pleased with him) said: Loans are of three kinds: 1. A loan by which you seek the pleasure of Allah, so you will have the pleasure of Allah. 2. A loan by which you seek the pleasure of your companion, so you will have the pleasure of your companion. 3. A loan that you lend in order to take something impure in exchange for something pure; that is usury. He said: O Abu Abd al-Rahman! Then what do you instruct me to do? He said: I think you should tear up the document. If he gives you the like of what you lent him, accept it; if he gives you less than what you lent him and you accept it, you will be rewarded; and if he willingly gives you back more than what you lent him, then that is gratitude with which he has thanked you, and you will have the reward for the respite you gave him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10937