Muwatta Imam Malik Book of Business Transactions, Hadith 93

And Malik related to me that he had heard that a man came to Abdullah ibn Umar and said, "Abu Abd ar-Rahman, I gave a man a loan and stipulated that he give me better than what I lent him." Abdullah ibn Umar said, "That is usury." Abdullah said, "Loans are of three types:
A free loan which you lend by which you desire the pleasure of Allah, and so you have the pleasure of Allah. A free loan which you lend by which you desire the pleasure of your companion, so you have the pleasure of your companion, and a free loan which you lend by which you take what is impure by what is pure, and that is usury." He said, "What do you order me to do, Abu Abd ar-Rahman?" He said, "I think that you should tear up the agreement. If he gives you the like of what you lent him, accept it. If he gives you less than what you lent him, take it and you will be rewarded. If he gives you better than what you lent him, of his own good will, that is his gratitude to you and you have the wage of the period you gave him the loan."

وَحَدَّثَنِي مَالِكٌ، أَنَّهُ بَلَغَهُ أَنَّ رَجُلاً، أَتَى عَبْدَ اللَّهِ بْنَ عُمَرَ فَقَالَ يَا أَبَا عَبْدِ الرَّحْمَنِ إِنِّي أَسْلَفْتُ رَجُلاً سَلَفًا وَاشْتَرَطْتُ عَلَيْهِ أَفْضَلَ مِمَّا أَسْلَفْتُهُ . فَقَالَ عَبْدُ اللَّهِ بْنُ عُمَرَ فَذَلِكَ الرِّبَا . قَالَ فَكَيْفَ تَأْمُرُنِي يَا أَبَا عَبْدِ الرَّحْمَنِ فَقَالَ عَبْدُ اللَّهِ السَّلَفُ عَلَى ثَلاَثَةِ وُجُوهٍ سَلَفٌ تُسْلِفُهُ تُرِيدُ بِهِ وَجْهَ اللَّهِ فَلَكَ وَجْهُ اللَّهِ وَسَلَفٌ تُسْلِفُهُ تُرِيدُ بِهِ وَجْهَ صَاحِبِكَ فَلَكَ وَجْهُ صَاحِبِكَ وَسَلَفٌ تُسْلِفُهُ لِتَأْخُذَ خَبِيثًا بِطَيِّبٍ فَذَلِكَ الرِّبَا . قَالَ فَكَيْفَ تَأْمُرُنِي يَا أَبَا عَبْدِ الرَّحْمَنِ قَالَ أَرَى أَنْ تَشُقَّ الصَّحِيفَةَ فَإِنْ أَعْطَاكَ مِثْلَ الَّذِي أَسْلَفْتَهُ قَبِلْتَهُ وَإِنْ أَعْطَاكَ دُونَ الَّذِي أَسْلَفْتَهُ فَأَخَذْتَهُ أُجِرْتَ وَإِنْ أَعْطَاكَ أَفْضَلَ مِمَّا أَسْلَفْتَهُ طَيِّبَةً بِهِ نَفْسُهُ فَذَلِكَ شُكْرٌ شَكَرَهُ لَكَ وَلَكَ أَجْرُ مَا أَنْظَرْتَهُ .