Riba (Usury) is of two major kinds:
Riba An-Nasia - Interest on lent money
Riba Al-Fadl - Taking a superior thing of the same kind of goods by giving more of the same kind of goods of inferior quality, eg.,dates of superior quality for dates of inferior quality in great amounts.
The definition of Interest, the literal meaning of interest or Al-Riba as it is used in the arabic language means to excess or increase. In the Islamic terminology interest means effortless profit or that profit which comes free from compensation or that extra earning obtained that is free of exchange. Riba has been described as a loan with the condition that the borrower will return to the lender more than and better than the quantity borrowed.
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http://muttaqun.com/riba.html