What is riba al-fadl, and is exchanging a mobile phone for another, or a car for another, with payment of the difference, considered riba al-fadl?

Riba al-Fadl is the sale of a ribawi item for its own kind with a surplus, as in the Hadith: "Gold for gold... like for like..." Exchanging a communication device for another with an increase, or a car for another, involves no sin, due to the absence of the علة الربا (underlying cause for Riba), such as thamanīyyah (being a currency), or ta'm (being edible), or kayl (being measurable by volume), or wazn (being measurable by weight) in them both.