Hadith: Whoever pays in advance for anything, let him pay for a specified measure and a specified weight, to be delivered on a specified period of time

عن عبد الله بن عباس رضي الله عنهما قال: قدم رسول الله صلى الله عليه وسلم المدينة، وهم يُسلفون في الثمار: السنة والسنتين والثلاث، فقال: «من أسلَفَ في شيء فليُسلف في كيل معلوم، ووزن معلوم، إلى أجل معلوم».
[صحيح] - [متفق عليه]

‘Abdullāh ibn ‘Abbās (may Allah be pleased with him) reported that when the Messenger of Allah (may Allah's peace and blessings be upon him) came to Madīnah, they used to pay one, two or three years in advance for fruits, so he said: "Whoever pays in advance for anything, let him pay for a specified measure and a specified weight, to be delivered on a specified period of time."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

When the Prophet (may Allah's peace and blessings be upon him) migrated to Madīnah, he found its residents, who were farmers, paying in advance the price of the fruits to be delivered within one, two, or three years. These terms are examples for the delivery periods, and they could be more or less. The Prophet (may Allah's peace and blessings be upon him) approved this transaction and did not consider it a sort of sale in which the seller sells what he does not own, which normally involves uncertainty. This is because paying for something in advance is mainly connected to the debts, not the merchandise. Nevertheless, the Prophet (may Allah's peace and blessings be upon him) ordained for them the rulings that govern such a transaction and keep them away from conflict and dispute that may arise from the long period of time after which delivery should be made. He said that whoever pays in advance for something, let him specify the measure and weight of that thing, and set a specific term for delivery. As long as the measure and term of delivery are known, there will be no chance for conflict or disagreement to take place, and each the parties involved can receive their rights peacefully.