of one portion for another, with any excess, is usury; hence it is impermissible to exchange an unknown amount for a known amount, or an unknown for another unknown, because equivalence cannot be assured and usury can seep into a small deal just as easily as into a large one.
According to Mālik, it is unlawful to sell a quantity of fine-eyed gold that is mixed with inferior gold for gold whose mint-mark is better than that of the inferior portion but lower than that of the superior portion, for the seller is in effect appropriating the extra value of the fine gold by means of the base gold he has added to it. If, however, the “eye” quality of the inferior gold mixed with the superior is itself better than—or equal to—the quality of the gold taken in exchange, the sale is unobjectionable, because all the surplus value then rests on one side.
In their estimation this falls under the example of “one sāʿ (dry measure) of wheat and a dirham for two sāʿs of wheat,” where the larger quantity is of medium grade, the medium is better than the inferior, and the deal is therefore not like-for-like but smacks of speculation and muzābana.
A number of scholars from Medina and elsewhere, however, allow such transactions, arguing that they are simply gold for gold or silver for silver, equal for equal, weight for weight. Their reasoning is that all gold—red or yellow, fine or coarse—constitutes a single genus that may only be exchanged on equal terms, and the same applies to silver, even though people may value them differently for various purposes.
If two parties are exchanging dirhams and one dirham proves deficient, the recipient may, before they part, take in its stead whatever food or other goods he wishes. Likewise, a person who owes half a dinar may hand over a full dinar: half of it settles the debt and the other half becomes a loan that the creditor may reclaim whenever he chooses, whether immediately or later, for no currency exchange is involved and nothing objectionable ensues.
It is impermissible for anyone to receive dinars in place of dirhams, or dirhams in place of dinars, before the due date—whether the original contract was a sale or a loan—and Mālik regarded it as reprehensible to receive from gold or silver—