because when you sell dirhams for dinars, possession must take place before the parties separate.

The consideration is apportioned between the two, and it is prohibited to sell over one’s brother’s sale—for example, for someone to say to a person who has bought an item for ten, “I will give you the like of it for nine.” Likewise, it is prohibited to purchase over his purchase—for example, for someone to say to a person who has sold an item for nine, “I will pay ten for it,” so that he may rescind [the first sale] and conclude a contract with him. The contract is invalid in both cases, ………

His statement, “The consideration is apportioned between the two,” means that the consideration is distributed between them proportionately when there is a need to do so.

An example is if you say, “I have sold you this car and leased you this house for one hundred thousand.” The price is one, and the contract is one. If it so happens that the house collapses—for instance, rains come and destroy it—the lease is terminated, because the subject matter of the contract has perished. This total consideration consists partly of rent and partly of the purchase price, so how is it to be apportioned?

The answer is that it is apportioned according to value. Thus, it is asked, “For how much can the house be rented?” If they say, “It can be rented for twenty thousand,” then the value of the car is eighty thousand. Accordingly, twenty thousand is deducted from the consideration. If, however, it is determined upon valuation that it is worth two hundred thousand, and they say that the value of the lease is twenty thousand while the contract was for one hundred thousand, we say: twenty thousand out of two hundred thousand equals 10 percent. We then apply this to the stated price of one hundred thousand and deduct 10 percent from it. This is what the author means by his statement, “The consideration is apportioned between the two”: if we need to distribute the consideration, it is apportioned according to the value of the item sold and the rental payment.

His statement, “It is prohibited to sell over his brother’s sale,” means that it is prohibited for a Muslim to sell over his brother’s sale, because the Prophet, peace and blessings be upon him, said, “None of you should sell over the sale of another” (1). It is also an act of wrongdoing against his brother, and because it leads to enmity, rancour, and estrangement. Everything that causes enmity and rancour among Muslims is prohibited. This is a general principle, based on Allah’s statement in explaining the prohibition of wine and gambling: “Indeed, Satan seeks only to sow enmity among you