What is the definition of Riba (usury) and the differences between its types, what is its permissible alternative, what are the non-usurious banks, and is buying Commercial International Bank (CIB) sukuk (Islamic bonds) that increase in value daily and can be sold at any time considered Riba or a variable interest? And is every variable interest permissible?

Usury (riba) is one of the grave sins forbidden by Islamic law. It means an increase in one of two homogeneous commodities without a مقابل (counter-value). It is of two types: Riba al-Nasi'ah (usury of delay), which is delaying payment in exchange for an increase, like bank interest; and Riba al-Fadl (usury of surplus), which is a mere increase without delay, like selling wheat for wheat with an increase. Islam has forbidden usury and has provided lawful alternatives such as selling (bay'), profit-sharing (mudarabah), and sukuk al-muqaradah (mudarabah certificates) used by Islamic banks.