an option to rescind, nor a guarantee, nor a transfer of liability is permissible. The exchange is valid only with proper immediate settlement: neither party may leave the other while any part of their transaction remains outstanding. The Messenger of Allah (peace and blessings be upon him) said:
“Do not sell gold for gold except in equal amounts, and do not make one amount exceed the other. Do not exchange any of it to be delivered later for that which is delivered immediately.”
‘Umar ibn al-Khattāb (may Allah be pleased with him) said: “Even if he asks you to wait until he enters his house, do not grant him that delay. I fear ramā for you.” Ramā means usury.
Section
Deferral in currency exchange falls into three categories. The first is where the two parties conclude the exchange on the condition that one grants the other time to deliver some of what they are exchanging, however little. If this occurs, the entire exchange must be annulled by agreement, because it was invalid from its inception.
(The second) is where they conclude the exchange on the basis of immediate settlement, but one subsequently grants the other time to deliver some of what they are exchanging. By agreement, the exchange is invalidated to the extent affected by the delay. If the deferred amount is one dirham, the exchange corresponding to one dinar is invalidated. This remains the case unless the deferred amount exceeds the equivalent of one dinar, in which case the exchange corresponding to two dinars is invalidated, and so on according to the same pattern.
There are two opinions on whether the portion of the exchange settled immediately, without any delay, remains valid. (The first) is that it does not, because the parties are suspected of having intended the deferral and contracted on that basis. This is Ibn al-Qāsim’s position in al-Mudawwanah and the opinion of Muhammad ibn al-Mawwāz. (The second) is that it remains valid and is not annulled. This is Ibn al-Qāsim’s opinion in the book of Ibn al-Mawwāz.
The third category is where they conclude the exchange on the basis of immediate settlement, but delivery of some of what is being exchanged is delayed through forgetfulness, error, or theft from