What is the ruling on selling the canceled currency notes (1000 and 500 rupees) for less than their face value, or on merchants accepting them for a lower value, given the sudden decision by the Indian government to cancel and replace them?

If old currency is still in circulation and used as a price for goods, it is impermissible to sell it for the same currency with a difference in amount. Rather, equality and immediate possession in the مجلس (gathering/session) are required, because it takes the ruling of gold and silver regarding the laws of riba (usury). However, if the currency is no longer in use and has ceased to be a price, then it becomes a commodity, and it is permissible to buy and sell it for the new currency with a difference in amount, because it is no longer considered currency.