Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) says: The salam sale, however its price is set, is usury, except when the measure is known and the term is known; and increase (the quantity) in such things as much as you can. In one narration of Abd al-Razzaq it is stated: "Make the salaf sale for a known weight and to a known term, and increase in such things as much as you can."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11117