We purchased 1 x 20 litre drum of oil which we sell in 1L quantities as they are used in jobs. Item was purchased as 1 item with 20 saleable units. Inventory states we have 20 in stock. I allocate 6 units to one job but cannot save it and get and error message stating i need to order 106 on backorder in order to save the invoice. Item has 20 selling units and 20 buying units wtih 20 in inventory. Why is this happening?