how to add a discount field many2one tax field in pos openerp7 already working discount and tax but i dont want that i need to add manually and calculate the total

More and more I'm having to throw money at OpenERP for every little thing. You're out of luck with the out-of-the-box. My advice, if you need to sell serial numbers and not products, then you need to use a different ERP. OpenERP is not built to support companies who sell serial numbers and not just products. Shame because it's a basic building block of any company who sells electronic equipment and it is a true crime to ignore these types of companies since there are so many. This feature should have been fleshed out when they tried adding serial numbers in the first place.

Sean: Serial number management at the item level is available. You need to activate the 2 first checkboxes in Settings > Warehouse; then create a new incoming shipment, receive the products, split in several serial numbers and that's where your traceability by serial starts. Using incoming shipments as the start is just an example; splitting a stock move in serial numbers is available in several places in the system.