What is the wisdom behind prohibiting the sale of debt for debt if it is permissible to sell a deferred item for a deferred price with a prior promise, and what is the difference between a contract and a promise in this case?

Firstly, scholars have unanimously agreed on the impermissibility of selling debt for debt. Among those who reported this consensus are Imam Ahmad, Ibn al-Mundhir, and Ibn Qudamah. The wisdom behind this prohibition is either that it leads to usury (riba), or that it falls under the category of speculation and gambling, or profiting from what is not guaranteed. However, selling an item for a deferred price, then selling it to another person after taking possession of it, whether for a deferred or immediate price, is permissible, as long as it does not fall under the forms of 'inah (a specific type of usurious transaction).

Secondly, deferred payment sales (بيع آجل) are permissible if the commodity is present and owned by the seller. If it is not owned, but the seller and buyer agree that the seller will first purchase it and then sell it on a deferred basis, there is no harm in that. An non-binding promise does not take the ruling of a sale. This is what the majority of jurists have ruled regarding murabaha sales for the one who orders the purchase. However, if the promise is binding on both parties, then it is forbidden.