The value of the unused stock is ₦350. This is because the opening stock of ₦1,800 plus the purchases of ₦2,800 gives us a total of ₦4,600. Subtracting the sales of ₦8,000 and the carriage on sales of ₦500, we have ₦3,100. Finally, subtracting the closing stock of ₦350 from the total gives us the value of the unused stock, which is ₦350.