All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
The conditions which the clerk of the Islamic bank must adhere to are as follows:
First: Receiving the commodity from the first seller and taking possession of it according to the rules of Sharia. This is in compliance with the Hadith of Ibn Abbas (Allah be pleased with them) who reported Allah's Messenger (PBUH) as saying: "He who buys food-rain should not sell it until he has taken possession of it. Ibn Abbas (Allah be pleased with them) said: I regard everything like food (so far as this principle is concerned)." {Agreed upon}.
Second: Paying the price of the commodity to the first seller and not authorizing the client ordering the purchase to do so.
Third: Non-signing of the Murabah contract of sale by the client ordering the purchase until the bank takes real or constructive possession of the commodity from the first seller. However, it is permissible for the client to sign the purchase promise.
Fourth: The bank clerk must make sure that contractual commitments, if any, between the client and the first seller are cancelled. But, if the client had only received a price proposal then this is permissible.
Fifth: The bank clerk must make sure that the first seller is a third party, other than the client or his proxy, to prevent the bank from falling a victim of fraud, or else this sale transaction is rendered invalid. And Allah The Almighty Knows Best.