All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If you are selling a product that is described but not specific [i.e. not seen by the customer] and a deadline for delivery is specified, then it is a Salam sale that is valid on the condition that the price is immediately paid in the contract meeting.
The Kuwaiti Fiqh Encyclopedia reads: “The Hanafi, the Ash-Shaafi'i, and the Hanbali Schools of jurisprudence, held that it is a condition that the capital (money) of the Salam sale be handed over in the meeting of the contract.” [End of quote]
Immediate payment of the price in the meeting of the contract occurs if one deposits the money in the account or by bank transfer; the resolution of the Islamic Fiqh Council reads:
“Among the forms of virtual possession [of the money] which are religiously considered, is when a person deposits an amount of money in the customer’s account directly or by a bank remittance.” [End of quote]
If it is difficult to fulfill the conditions of the Salam sale, and you want to render the transaction valid, then you either be an agent for the seller in the sale for a known fee, or that you only promise to the buyer and then you buy the product and then sell it to him.
[The definition of Salam sale: a contract according to which the price of a clearly defined item is paid in advance at the time of concluding the contract, and the item is delivered at a later time.]
For more benefit on the definition, conditions, and rulings of the Salam Sale contract, please refer to Fataawa 11368, 27508, and 378148.
Allah knows best.