What is the operative cause for classifying salt among the categories of riba (usury or interest), given that it is not a staple food?

The questioner : May Allah reward you. Regarding salt.

The Shaykh : May Allah have mercy on you. Huh?

The questioner : ... Regarding salt, we say that it is measured by volume, but it is not needed for... the operative cause... .

The Shaykh : Yes, because it is a staple food.

The questioner : Well, we said that both operative causes must be present.

The Shaykh : Yes.

The questioner : ... .

The Shaykh : The scholars say that this is suitable for preparing staple food, so it has its ruling.

The questioner : ... the operative cause... .

The Shaykh : The operative cause? No, it is not for storage, but this is suitable for preparing staple food because people cannot cook food without salt in it.

The questioner : Shaykh.

The Shaykh : Yes, Khalid?