Ibn ʿAbbas said, "Messenger of Allah ﷺ forbade the selling of foodstuff before its measuring and transferring into one's possession." I asked Ibn ʿAbbas, "How is that?" Ibn ʿAbbas replied, "It will be just like selling money for money, as the foodstuff has not been handed over to the first purchaser who is the present seller."
أَنَّ رَسُولَ اللَّهِ ﷺ نَهَى أَنْ يَبِيعَ الرَّجُلُ طَعَامًا حَتَّى يَسْتَوْفِيَهُ قُلْتُ لاِبْنِ عَبَّاسٍ كَيْفَ ذَاكَ قَالَ ذَاكَ دَرَاهِمُ بِدَرَاهِمَ وَالطَّعَامُ مُرْجَأٌ قَالَ أَبُو عَبْد اللَّهِ مُرْجَئُونَ مُؤَخَّرُونَ