According to the orthodox, an "increase over the principal sum" in "loans of cash" are riba. An increase over the principal sum in "financing a purchase" of some product or commodity is another matter. These are "not" riba — according to the orthodox interpretation — at least in some circumstances. (These are sometimes known as "credit sales".) According to noted Islamic scholar Taqi Usmani, this is because in Quran aya 2:275 (""they say, 'Trafficking (trade) is like usury,' God has permitted trafficking, and forbidden usury"") "trafficking (trade)" refers to credit sales such as "murabaha", the "forbidden usury" refers to charging extra for late payment (late fees), and the "they" refers to non-Muslims who did not understand why if the first was allowed both were not. For this reason (according to Usmani) it is "not" true that "whenever price is increased taking the time of payment into consideration, the transaction comes within the ambit of interest". Instead of "principal" and "interest rate", the credit taker is paying "cost" and "profit rate". (Another difference with conventional finance is that there is no penalty for late payment.)