When the Messenger of Allah ﷺ came to Medina, they were paying one, two and three years in advance for fruits, so he said: Those who pay in advance for anything, must do for a specified measure and weight with a specified time fixed.  

قَدِمَ رَسُولُ اللَّهِ ﷺ وَهُمْ يُسْلِفُونَ فِي التَّمْرِ السَّنَةَ وَالسَّنَتَيْنِ وَالثَّلاَثَةَ فَقَالَ رَسُولُ اللَّهِ ﷺ مَنْ أَسْلَفَ فِي تَمْرٍ فَلْيُسْلِفْ فِي كَيْلٍ مَعْلُومٍ وَوَزْنٍ مَعْلُومٍ إِلَى أَجَلٍ مَعْلُومٍ  

'Abd Allah b. Shaddad and Abu Burdah disputed over salaf (payment in advance). They sent me to Ibn Abi Awfa and I asked him (about it) and he replied: We used to pay in advance (salaf) during the time of the Messenger of Allah ﷺ, Abu Bakr and 'Umar in wheat, barley, dates and raisins. Ibn Kathir added: "to those people who did not possess these things." The agreed version then goes: I then asked Ibn Abza who gave a similar reply.  

إِنْ كُنَّا نُسْلِفُ عَلَى عَهْدِ رَسُولِ اللَّهِ ﷺ وَأَبِي بَكْرٍ وَعُمَرَ فِي الْحِنْطَةِ وَالشَّعِيرِ وَالتَّمْرِ وَالزَّبِيبِ زَادَ ابْنُ كَثِيرٍ إِلَى قَوْمٍ مَا هُوَ عِنْدَهُمْ ثُمَّ اتَّفَقَا وَسَأَلْتُ ابْنَ أَبْزَى فَقَالَ مِثْلَ ذَلِكَ  

"to those people who did not possess these things." Abu Dawud said: What is correct is Ibn Abi al-Mujahid. Shu'bah made a mistake in it.  

عِنْدَ قَوْمٍ مَا هُوَ عِنْدَهُمْ  

We made a journey to Syria on an expedition along with the Messenger of Allah ﷺ. The Nabateans of Syria came to us and we paid in advance to them (in a salam contract) in wheat and olive oil at a specified rate and for a specified time. He asked (by the people): you might have contracted with him who had these things in his possession? He replied: We did not ask them.  

غَزَوْنَا مَعَ رَسُولِ اللَّهِ ﷺ الشَّامَ فَكَانَ يَأْتِينَا أَنْبَاطٌ مِنْ أَنْبَاطِ الشَّامِ فَنُسْلِفُهُمْ فِي الْبُرِّ وَالزَّيْتِ سِعْرًا مَعْلُومًا وَأَجَلاً مَعْلُومًا فَقِيلَ لَهُ مِمَّنْ لَهُ ذَلِكَ قَالَ مَا كُنَّا نَسْأَلُهُمْ