The Official Website for The G.A. Wahid Khorasani

The Conditions for an Advance Payment Transaction
2140. An advance payment transaction must fulfill the following seven conditions:
1. The details which determine the price of the commodity should be specified, and it will be sufficient to specify it to an extent that it would be accepted in the common understanding that its details have been made known. As for the commodities whose specifications and characteristics cannot be defined except by visual observation, such as precious stones and similar items, advance payment transactions are not valid in their case.
2. Before the buyer and the seller leave each other’s presence, the buyer should pay the entire price to the seller, or be owed by the seller an amount that is equivalent to the price—and the amount is either owed immediately, or in the form of deferred payment whose period has come to an end—and thus offer to adjust the debt against the price, to which the seller accepts.
However, if the buyer only pays a part of the price, the transaction will be valid with respect to that amount, and the seller reserves the right to cancel the transaction.
3. The period should be specified in a very precise manner. If for example he says, “I will deliver the commodity at the beginning of the harvest,” the transaction will be invalid because the period has not been precisely defined.
4. The time specified for the delivery should be such that the seller would be able to deliver the commodity at that time.
5. A place for the delivery should be specified in the event that varying locations may cause difficulty in submitting the commodity or loss of property. However, if the delivery location is obvious due to certain qualifiers, it is not necessary to name the location.
6. The amount of the commodity should be specified by weight, volume, count or any such measurements. If the commodities that are usually sold by visual observation, are sold by taking advance payment, there is no problem in it. However, the difference within the commodity should be so insignificant, that people would not pay attention to it, as is the case with certain kinds of eggs and walnuts.
7. If the commodity being sold is usually sold by weight or volume, what is acquired in return should not be of the same commodity. For example, wheat cannot be sold for wheat by taking advance payment.