Odoo - Subtract weight/qty on product in POS

In a shop whre you sell food by scale/weight it would be nescsary to remove the cost of the packaging/container the food is in when you put in on the scale. We want a module that alow us to enter a weight/qty on products, that should be subtractet from the value that the POS system get from the scale. Ex: In the product "food on a plate" we enter 0.500 in a "subtract" field. When we are in the POS mode, and select product "food on plate" and put the plate on scale, the scale shows 1.400 but in post it will show 0.900.