Chapter on Usury (Riba)
Man Lā Yaḥḍuruh al-Faqīh|Volume 3|Book 2|Chapter 27
وَ سَأَلَهُ سَمَاعَةُ: عَنِ اَلطَّعَامِ وَ اَلتَّمْرِ وَ اَلزَّبِيبِ فَقَالَ "لاَ يَصْلُحُ شَيْءٌ مِنْهُ اِثْنَانِ بِوَاحِدٍ إِلاَّ أَنْ تَصْرِفَهُ مِنْ نَوْعٍ إِلَى نَوْعٍ آخَرَ فَإِذَا صَرَفْتَهُ فَلاَ بَأْسَ بِهِ اِثْنَانِ بِوَاحِدٍ وَ أَكْثَرَ مِنْ ذَلِكَ".
Sama'ah asked him about food, dates, and raisins. Imam عليه السلام said: "It is not permissible to exchange two for one of any of these, except if you change it from one type to another. When you change it, then there is no harm in exchanging two for one or even more than that."
- Ṣaḥīḥ al-Faqīh, p. 246