Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says: To make a sale saying, "I will pay when the stipend comes, or when the crop is harvested, or when the threshing floor is set up, or when the grapes are pressed," such a sale is not permissible; rather, a term should be fixed.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11115
Ikrimah (may Allah have mercy on him) says about Ibn Abbas (may Allah be pleased with them both) that he regarded as disliked a salam sale made until the harvesting of the crop, or until it has fully grown, or until the crop is on the threshing floor, and he would say: "Fix the term by naming a month."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11116
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
Abu Ubaydah (may Allah have mercy on him) narrates from his father (may Allah be pleased with him) that he did not consider it permissible to sell wheat at the threshing-floor rate.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11118
Amr bin Dinar says: Abdullah bin Umar (may Allah be pleased with them both) disliked that a man should buy something and say: "I will pay the price when I have the means."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11119
Kulayb bin Wa'il says: I said to Ibn Umar (may Allah be pleased with them both): "I had some dirhams to collect from a certain man. I went to him to collect them, and he said: 'I have nothing at present, but write it down against food until the harvest.'" Ibn Umar (may Allah be pleased with them both) said: "That is not proper."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11120
Sayyidah Aishah (may Allah be pleased with her) says: Some traders came with goods. I said to the Messenger of Allah (peace and blessings be upon him): "O Messenger of Allah! Stop wearing these old clothes and buy new ones, and pay for them when you have the money." The Prophet (peace and blessings be upon him) sent a message to that trader: "Give me two garments; I will pay you their price in days of ease." He said: "Muhammad wants to consume my wealth." He (peace and blessings be upon him) said: "They know well that I am the most trustworthy and the most God-fearing of them all."
This is to be understood as meaning that he made an offer, but the trader refused; he had not concluded the sale. And even if he had agreed, it is more likely that he (peace and blessings be upon him) would have fixed a time, or made an unconditional sale and then paid when he had the means.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11121