The items in which riba (usury or interest) applies

Question:
He says: Riba applies to six categories; as for the six categories other than these, you may sell however you wish. This is a testimony.

Answer:
If he chooses the names, provided it is hand-to-hand, as the Messenger (peace and blessings be upon him) said: One item from this category is not to be sold for an increased amount of the other. However, if he sells it for the other hand-to-hand, there is no harm. Gold for silver hand-to-hand, wheat for barley hand-to-hand, dates for salt hand-to-hand, there is no harm. For this reason, he said: If he chooses the names, how does he name them if it is hand-to-hand? And so on, whatever resembles wheat is included with wheat, such as corn, millet, and rice, which are like it. And so on, whatever resembles gold and silver is included with them, such as the weighed items.