They replied:
We have two possible interpretations. The first is that the “two parties to a sale” mentioned in the ḥadīth are merely bargaining with one another; the sale has not yet been concluded between them.

It was objected to them: On that reading the ḥadīth conveys no benefit at all, for it is common knowledge in the religion of the ummah that both parties remain free to withdraw so long as no verbal contract has been concluded.

As for the second interpretation, they said: The “separation” (tafarruq) spoken of here is a figure of speech for parting by words, not for physically dispersing, just as Allah, Exalted is He, says: “But if they separate, Allah will enrich each of them out of His abundance” (al-Nisāʾ 4:130).

The rejoinder is that this sense is metaphorical rather than literal, whereas the literal meaning is physical separation. The correct approach is to compare the apparent wording with analogy and give precedence to whichever is stronger. The wisdom in doing so lies in allowing room for regret. These, then, are the foundations of the first pillar, namely the contract itself.

[Chapter Two: The Subject Matter of the Contract]
Concerning the second pillar—the object of the contract—it is a condition that it be free of gharar (uncertainty) and ribā (usury). The points of agreement and disagreement on this, together with their causes, have already been discussed, so there is no need to repeat them. Gharar is removed when the item’s existence, qualities, and quantity are all known, and when delivery is within the seller’s power; the same applies to both sides of the exchange, price and merchandise. If the sale is deferred, the term must also be known.

[Chapter Three: The Contracting Parties]
As for the third pillar—the two contracting parties—it is required that each of them be a full owner, or a fully authorized agent empowered to transact even with some latitude in pricing; that neither of them be under interdiction, whether for their own sake—such as a spendthrift according to those who hold he may be interdicted—or for the sake of another, as in the case of a slave, unless the slave has been permitted to trade.

From this arises the dispute about the sale carried out by an unauthorized person (bayʿ al-fuḍūlī): does it take effect or not? The case is that a man sells another’s property on condition that if the owner approves the sale is ratified, and if he does not it is revoked; likewise a man may purchase for another without his permission, on condition that if the would-be purchaser accepts, the purchase is valid, otherwise it is not. Imām al-Shāfiʿī rejects both situations, Mālik permits both, while Abū Ḥanīfah distinguishes between selling and buying, holding that selling is permissible but buying is not.

The Mālikīs’ principal evidence is the report that the Prophet—may Allah bless him and grant him peace—handed ʿUrwah al-Bāriqī a dinar and said, “Buy us a sheep from that newly arrived stock.” ʿUrwah relates: “I bought two sheep for a dinar, then sold one of the two for a dinar, and brought the sheep and the dinar back. I said, ‘Messenger of Allah, here is your sheep and your dinar.’ He replied, ‘O Allah, bless him in the transactions of his right hand.’”

The force of the argument is that the Prophet—peace and blessings be upon him—did not instruct him regarding the second sheep, neither to buy nor to sell. Thus the ḥadīth is a proof against Abū Ḥanīfah concerning the validity of purchasing on behalf of another, and against al-Shāfiʿī with respect to both buying and selling.