3. It is permissible to sell gold for silver, or silver for gold, with an imbalance between the two amounts, because each of them belongs to a different genus.
   Likewise, whenever one riba-bearing genus is exchanged for something outside its own kind, disparity between the two is unobjectionable.

4. In every sale of gold for silver, or silver for gold, both sides must take possession during the contract session. If they part before possession takes place, the contract is null, for the two counter-values share the operative cause of ribā. The same applies to any two commodities that have the same riba-bearing cause—namely, items sold by measure or by weight that are also edible: immediate mutual possession in the contract session is indispensable.

Scholarly disagreement over banknotes

In recent times people have set aside gold and silver and begun trading with banknotes (anwāṭ). Each currency now has its own denominations, bearing its name and value. Scholars have therefore differed concerning their legal status; here is a brief outline of their opinions.

1. Some regard banknotes as nothing more than promissory notes, debts, or certificates, and so they forbid all transactions involving them outright.

2. Others treat them as ordinary merchandise, claiming that neither form of ribā applies to them. This lenient view is the opposite extreme of the first. Its proponents allow notes to be exchanged for other notes, or for either precious metal, with disparity and even on credit, arguing that no ribā occurs.

Both of these views are exceedingly weak.

• The first is unduly strict, placing people in hardship and constraint, whereas the nature of our religion is facilitation and ease—especially in matters of custom and commerce.
• The second opens the door to a grave evil, namely ribā in all its forms, and it rests on no sound legal reasoning.

3. A third group maintains that banknotes share the same ruling as the two precious metals; all regulations that apply to gold and silver, they say, apply equally to banknotes. This opinion has some merit, given the strength of its basis; its advocates reason that whatever stands in for something else inherits its ruling in every respect.

The finest, most balanced, and—God willing—most accurate view is to treat banknotes as fulūs (copper coins). Thus, ribā al-nasīʾah (deferred usury) applies to them, whereas ribā al-faḍl (excess in a spot transaction) does not. Accordingly, it is lawful to sell some notes for others, or for either precious metal, with a disparity that reflects their monetary value; disparity in the physical notes themselves is of no consequence. Such transactions, however, must never be conducted on credit.