What is the ruling on the following financial transactions: transferring money by agreeing on the price before or after the amount arrives, selling dollars for future delivery at a higher price than the market rate, and buying post-dated checks for less than their current value?

The exchange of currencies (money changing) stipulates, for its permissibility, that the exchange takes place hand-to-hand, and that it be like for like when dealing with a single currency. Disparity in the same currency is usury of surplus (riba al-fadl), and delaying possession is usury of deferment (riba al-nasi'a), both of which are forbidden.

Thus, the transaction is valid if the exchange takes place in cash between the seller and the customer, and the subsequent delay in the arrival of the transferred amount does not cause harm. However, if the exchange does not take place immediately, the transaction is not permissible as it involves usury of deferment.