Riba

The definition of riba in classical Islamic jurisprudence was "surplus value without counterpart." or "to ensure equivalency in real value" and that "numerical value was immaterial."

During this period, gold and silver currencies were the benchmark metals that defined the value of all other materials being traded.

Applying interest to the benchmark itself (ex natura sua) made no logical sense as its value remained constant relative to all other materials: these metals could be added to but not created (from nothing).

Applying interest was acceptable under some circumstances. Currencies that were based on guarantees by a government to honor the stated value [“fiat money”] or based on other materials such as paper or base metals were allowed to have interest applied to them When base metal currencies were first introduced in the Islamic world, no jurist ever thought that "paying a debt in a higher number of units of this fiat money was riba" as they were concerned with the real value of money (determined by weight only) rather than the numerical value.

For example, it was acceptable for a loan of 1000 gold dinars to be paid back as 1050 dinars of equal aggregate weight (i.e., the value in terms of weight had to be same because all makes of coins did not carry exactly similar weight).