What is the definition of Riba (usury), and is accepting payment in the currency of countries that are founded on the principle of capital circulation and lending considered aiding a usurious system, and does using the currency of these countries constitute contributing to their usurious economy?

Usury (riba) is an increase in a specific thing. Its origin in pre-Islamic times was the increase of a debt upon its maturity. The Shari'ah prohibited another type of usury, which is riba al-fadl (usury of surplus), which is the increase in one of two commodities when selling one for the other, such as selling gold for gold without equality and immediate possession.

The principle of capital circulation, represented by borrowing with an increase upon repayment, is forbidden usury. As for the circulation of capital in trade through profit-sharing between the capital owner and the worker (Mudarabah), there is no harm in it.

Depositing money in interest-based banks and taking interest from them is forbidden, and it is not permissible to deal with these banks or to borrow with an increase. Using internationally recognized currency (such as the dollar) is permissible when there is a need, even if it contributes to the economy of its country, because it is globally recognized, and it can be dispensed with if Islamic currencies are available.