What is the meaning of "like for like" in the Hadith concerning currency exchange, and what is the benefit of exchanging equivalents in weight if neither of the two substitutes is better than the other?

The Sunnah has prohibited riba al-fadl (usury of surplus) in specific categories such as gold, silver, wheat, barley, dates, and salt. The sale of these items must be "like for like, equal for equal, hand to hand."

No rational person would exchange something for another identical item unless there is a difference in qualities, such as good and bad.

The wisdom behind the prohibition of riba al-fadl is that it serves as a means (dhari'a) to riba al-nasi'a (usury of delay). Furthermore, usurious monies (like dirhams and dinars) are prices and standards for evaluating wealth, so they should not become commodities to be traded, thus losing their function. Also, trading in foodstuffs (like wheat and dates) leads to their monopolization and withholding from those in need, which harms people.

The ultimate goal is that these categories should not be traded for their inherent nature. If they are traded for their nature, it should be without profit and without delay.