Klasik dönemde hakim olan faiz teorileri günümüzde aynen tatbik edildiklerinde, kağıt paraların ya da bir çok sanayi ürününün kendi cinsleriyle mübadelesinde eşitliğe riayet edilmesi gerekmediği sonucu açığa çıkmaktadır. Sözgelimi, fazlalık faizinde bedeller arasında cins birliği yanında keylîlik ya da veznîlik birliği aranan teoriye göre, keylî ya da veznî olmayan kağıt paralar ve bir çok sanayi ürünü fazlalık faizinin konusu değildirler. Bedellerin yiyecek maddesi, altın ya da gümüş olmasını şart koşan diğer hakim faiz teorisi esas alındığında da aynı sonuçla karşılaşılmaktadır. Buna karşılık, günümüzde kağıt paraların ve birbirine eşit olarak üretilen sanayi ürünlerinin kendi cinsleriyle mübadelelerinde eşitliğin mutlaka şart koşulması gerektiği düşüncesiyle çağdaş İslam Hukukçuları, çoğunlukla, faiz yasağının kapsamını yeni bir faiz teorisi ortaya koyarak genişletme eğilimi içerisindedirler. Bu nedenle çağdaş dönemde, klasik dönemde hakim olduğu belirtilen teoriler yerine, her türlü paranın ve her türlü mislî malın kendi cinsleriyle mübadelelerinde eşitliğe riayet edilmesini gerektiren yeni bir teori yaygınlık kazanmaya başlamıştır. Sözü edilen durum, aynı zamanda klasik teorilerin günümüzde geçerliliğini sürdürüp sürdürmediği sorununu açığa çıkarmaktadır. Bu araştırmada, gerçekte kağıt paraları ve tüm sanayi ürünlerini kapsayan bir genişlikte olduğu gerekçesiyle, Hanefî faiz teorisi bakımından günümüzde herhangi bir geçerlilik sorununun bulunmadığı gösterilmek istenilmektedir.  

In the classical period, it is seen that two different theories about riba’l-fadl (surplus interest) have become widespread. According to the first theory that belongs to Hanafi and Hanbali scholars, genus and qadr (kayl/vazn) unity among goods is stipulated. For example, the fact that the surplus of gold is forbidden in exchange of gold stems from its being weighable, not from that gold is money. And the fact that the surplus of wheat is forbidden in exchange of wheats stems from its being measurable. Therefore, riba’l-fadl does not accrue in the exchange of unmeasurable or unweighable goods with their own type. Equality is not required in the exchange of ‛adadî goods (which their quantities are known by number) such as apples and eggs or zar‛î goods (which their quantities are known by length measure) such as hand-woven fabrics, rugs and so on with their own kinds. According to the second theory that belongs to Shafi‛i and Maliki scholars, both of goods in an exchange must be foodstuff, gold or silver. Therefore, equality does not need to be observed in the exchange of non-gold or non-silver coins or non-food commodities such as cotton, wood, iron and brick with their own species. When the common riba theories in the classical period are applied exactly today, it is revealed that the excess in exchange of banknotes or many industrial products with their own kinds is not forbidden  For example, according to the riba’l-fadl theory based on genus and qadr (kayl/vazn) unity among goods, paper money and many industrial products that are not measured with kayl or vazn are not subject to the riba’l-fadl. A similar result is seen when we consider the other common riba’l-fadl theory that requires goods to be food or gold or silver. Contemporary Islamic jurists, generally, with the idea that equality must be stipulated in the exchange of paper money and industrial products which are produced equally with each other, tend to extend the scope of the riba prohibition by introducing a new riba theory. Therefore, in the contemporary period, instead of the theories that were dominant in the classical period, a new theory began to become widespread, requiring that all kinds of money and all kinds of property should be exchanged equally with their own kinds. These scholars argue that ‛illa must be al-samaniyya al-mutlaka in gold and silver, and al-misliyya al-mutlaka in other products. The acceptance of these two qualities as an ‛illa requires the consideration of equality in the exchange of all kinds of money and all kinds of property with its own. However, the following should be taken into consideration: The claim that ‛illa is al-samaniyya al-mutlaka in gold and silver, and al-misliyya al-mutlaka in other products, it also raises the claim that either none of the theories prevailing in the classical period represented truth or that classical theories representing truth in the past are no longer valid. Therefore, the question of whether classical interest theories are still valid today is a serious problem. This problem is discussed in detail in this study in terms of the Hanafi riba theory. When classical Hanafi interest theory is examined, it is understood that the reason of the consideration of genus and qadr (kayl/vazn) unity among goods in Hanafi Theory is the fact that the only way to establish an exact equivalence between goods is with kayl or vazn in that periods. It was not possible to fully determine whether ‛adadi or zar‛i goods are equivalent to each other. They were never seen as exactly equal to each other in that period. In the classical period, it is generally mentioned from inequality or similarity not exact equality among ‛adadi or zar‛i goods. However today, the situation is quite different. Since the industrial revolution, the ‛adadi or zar‛i goods with full equality has emerged and has become increasingly widespread. Therefore, since the industrial revolution, according to the Hanafi interest theory which adopts the principle of establishing equality between goods in an exchange, it is necessary to take ‛adadi or zar‛i goods which are exactly equal into interest goods. In other words, it is necessary to accept qualifications of being countable or measurable by length as an ‛illa when they are exactly equal to each other. Indeed, as far as Abdulaziz al-Buhari quotes, Imam al-Burgari clearly states that exact equal countablity in the Hanafi theory is considered as an ‛illa about coins in circulation. Consequently, there is no validity problem in terms of the Hanafi theory, since it actually covers paper money and all industrial products.