What is the ruling on usury of delay (riba al-nasi'ah) in non-usurious commodities, and what are the sayings and schools of thought of scholars regarding the sale of a non-usurious commodity of the same kind with disparity and delay?

Summary of the Answer from the Kuwaiti Fiqh Encyclopedia:

Ribawi items (items subject to usury) are of two types: those explicitly mentioned in texts, and those in which the illah (underlying reason or effective cause) for riba is present.

The Shafi’is and the most authoritative view among the Hanbalis hold that riba is not prohibited in items other than these. Therefore, it is permissible to sell some of these items for others with tafāḍul (disparity in quantity) and nasīʾah (deferred payment), and it is permissible to separate before taqābuḍ (mutual possession). They cite the actions of the Companions, such as Ibn Amr, Ibn Abbas, Ali, and others, who sold animals for animals with tafāḍul and nasīʾah.

The Hanafis and, in one narration, the Hanbalis, prohibited selling an item of its own kind with nasīʾah, such as animal for animal, based on the hadith (definition) of Samurah: "He [the Prophet] forbade selling an animal for an animal with nasīʾah."

The Malikis hold that riba can be conceived in commodities and animals other than currencies and foodstuffs if three characteristics are combined: tafāḍul (disparity), nasīʾah (deferment), and agreement in purposes and benefits.