jurisprudence to determine what commodities were subject to riba (i.e. being able to be measured, eaten or used as legal tender) ignores reasons why a sale should be prohibited (hikmah) -- issues such as "the circumstances of the transaction, the parties thereto, or the importance of the commodity to the survival of society." Mahmoud El-Gama notes that orthodox interpretation (or at least orthodox Hanafi) of riba (the basis of what he attacks as "shari'a arbitrage") distinguishes between fungible (mithli) and non-fungible (qimi) items. Thus (allegedly) fungible gold may not be traded one ounce for two, but trading one non-fungible item (such as diamonds) for two is permitted, whatever the items' market value. Thus "selling a diamond worth $10,000 today for a deferred price of $20,000 tomorrow" and immediately selling the diamond for $10,000 in cash is halal (legal) under orthodox rules of riba al-fadl — notwithstanding the fact that it would give the financier an effective rate of 100% interest. El-Gama describes this as avoiding "riba in form" while being "usurious in substance". Rationale According to Abdullah Saeed, "the intended meaning" of the ahadith concerning riba al-fadl "was not very clear even to many jurists", who nonetheless believed the prohibition "was to be observed and complied with ... without probing into the reasons for the prohibition." Other scholars have probed. Ibn Rushd stated that "what is targeted by the prohibition of riba is the excessive inequity it entails". Taqi Usmani asserts that Riba al-fadl was developed by Muhammad after his ban on riba to avoid "certain barter transactions might lead the people to indulge in Riba", picking out commodities that were "a medium of exchange like money". Iqbal Suhail believes trading lesser quality foodstuffs for better quality and less quantity was forbidden because the frugality and austerity of Muhammad was offended by something like the spending resources on higher quality foodstuffs "for the sake of gratification of the palate." Others believe riba al-fadl makes little sense as a prohibited sin but does as a sort of consumer advice. Mohammed Fadel (of the faculty of law, University of Toronto) calls it a ‘prudential regulation’. Farooq suggest it may have arisen to warn Muslims that barter is usually less profitable than buying and selling separately, and notes several hadith where Muhammad tells a Muslim not to trade dates of different quality but never mentions riba. M.A. Khan argues that the prohibition against