The sale of a debt (dayn) for a debt (or the exchange of debt for debt). This type of sale is prohibited under shari’a because it involves the exchange of deferred forms of wealth (nasi’ah). An example of a prohibited ba’i al-dayn bil dayn is a salam transaction in which payment is not made by the purchaser at the time both parties enter into the contract of salam. Fuqaha held four different views on the permissibility of this sale:

• Complete prohibition: Hanafis and Hanbalis embraced this stance because they prohibited this type of sale in all its forms, whether the debt was sold to the debtor or to a third party. In their opinion, ba’i al-dayn bil dayn was invalidated based on the Prophetic maxim (hadith) in which ba’i al-kali bil kali was categorically prohibited. However, some Hanafi scholars permitted it on the condition that the two countervalues (badalain) are actually or constructively exchanged at the time of the transaction, and that both countervalues are determinable or specifiable.
• Permissibility: this is the opinion of Shafiis who stipulated for permissibility that the sale be entered into with the debtor himself and that the countervalues be specifiable (not unknown). They also stipulated that the countervalues are different in terms of genus such as dollar for euro, cotton for wool, rice for dates, etc.
• Conditional permissibility: Malikis permitted this sale with some conditions including: specification of countervalues (even if they were usufructs) and acceleration of payment (thaman) in absolute debts.
• Unconditional permissibility: other fuqaha (such as Ibn Taimyyah and Ibn al-Qayym) permitted the sale of a debt for a debt in all circumstances- i.e., whether to the debtor himself or to a third party.

In view of these varying perspectives, preponderance can be given to general permissibility provided that the following conditions are appropriately met:

• The price (thaman) must be specifiable, whether it takes the form of tangible assets (ayn) or usufructs (manfa’ah).
• The object of sale and the consideration (badal) must not be ribawi items.
• The sale transaction must be carried out by exchanging the two countervalues (badalain), actually or constructively.

This type of sale is also known as ba’i al-kali bil kali.