Quantity is in the outer Select statement which only returns results from the products table.
Your query is wrong and I could only guess without knowing your table structure.... Maybe what you are going for is:
SELECT Quantity - (SELECT Coalesce(SUM(DetailQuantity),0) FROM orders Inner Join orderdetails ON orderdetails.DetailOrderID = orders.OrderID WHERE OrderDate > ProductUpdateDate AND DetailProductID = Products.ID) AS RemainingInventory FROM products INNER JOIN ProductsBatch on products.ProductID = ProductsBatch.ProductID WHERE ProductID = %s