From the Matters of As-Salam that is permissible

Question :

If there is a person in need and he takes a sum of money from one of the people under the condition that he will give it back to him after a specified period of time in Sa's of wheat or measures of fruit with that being before they are ripe, is this allowed?

Answer:

If he adheres to it with the mentioned measures that are in his possession, then this matter is considered from the matters of As-Salam (pay in advance for goods received later). As-Salam is a type of sale that is correct with its conditions along with seven basic conditions:

The first: That it be from those things that can be described in specific.

The second: That he describes it according to what would obviously cause the price to differ.

The third: That he mentions its amount by measurement for those things that are measured, by weight for those things that are weighed and by length for those things that are measured by length.

The fourth: That he stipulates a condition for the item to be delivered at a specified later date.

The fifth: That the submitted item be generally present at his place (shop, farm, etc.).

The sixth: That the price is paid at the sitting of the agreement (up front).

The seventh: That the person pays in advance for a product in possession. If money is paid for a product that is not owned by the seller, the transaction is not correct.

The basis for the permissibility of As-Salam in the Qur'an is Allâh's Statement:
"O you who believe, when you contract a debt for a fixed period, write it down."

Ibn 'Abbas said, "I bear witness that As-Salaf (payment in advance, i.e., As-Salam) that is guaranteed for products to be received later has been made lawful by Allâh, the Most High, in His Book and He has allowed it." Then he recited this Verse. This narration was reported by Sa'id.

From the Sunnah there is what was reported by Ibn 'Abbas that when the Prophet (first) came to Al-Madinah, the people were practicing As-Salaf (i.e., As-Salam) with fruit to be delivered after a year or two. So he said:
"Whoever pays in advance for something, then let him pay in advance for a known (i.e., specified) measurement, a specified weight and for a specified period of time."

This narration is agreed upon. May Allah send blessings and peace upon Muhammad, his family and his Companions.