It was narrated that Ibn 'Abbas said:
"When the Prophet ﷺ came (to Al-Madinah), they used to pay in advance for dates, two or three years in advance. He said: 'Whoever pays in advance for dates, let him pay for a known amount or a known weight, to be delivered at a known time.’”

حَدَّثَنَا هِشَامُ بْنُ عَمَّارٍ، حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، عَنِ ابْنِ أَبِي نَجِيحٍ، عَنْ عَبْدِ اللَّهِ بْنِ كَثِيرٍ، عَنْ أَبِي الْمِنْهَالِ، عَنِ ابْنِ عَبَّاسٍ، قَالَ قَدِمَ النَّبِيُّ ﷺ وَهُمْ يُسْلِفُونَ فِي التَّمْرِ السَّنَتَيْنِ وَالثَّلاَثَ فَقَالَ " مَنْ أَسْلَفَ فِي تَمْرٍ فَلْيُسْلِفْ فِي كَيْلٍ مَعْلُومٍ وَوَزْنٍ مَعْلُومٍ إِلَى أَجَلٍ مَعْلُومٍ " .