The Definition Of Riba
It is Riba when it is a loan transaction, not on the spot. The principle is described in the following Hadith: الْوَرَقُ بِالذَّهَبِ رِبًا إِلَّا هَاءَ وَهَاءَ وَالْبُرُّ بِالْبُرِّ رِبًا إِلَّا هَاءَ وَهَاءَ وَالشَّعِيرُ بِالشَّعِيرِ رِبًا إِلَّا هَاءَ وَهَاءَ وَالتَّمْرُ بِالتَّمْرِ رِبًا إِلَّا هَاءَ وَهَاءَ If you lend gold in exchange for silver, then there is a possibility of interest in this. Similarly, for wheat in exchange for another type of wheat, barley in exchange for another type of barley, date for another type of date. Indeed if the exchange is done on the spot, then there is no harm. Muslim,Al-Jami‘ al-sahih, 692, (no. 4059).