My question is almost identical to this one: http://stackoverflow[DOT]com/questions/18048496/calculate-available-inventory-in-openerp

So i'll just quote him:

"I am developing a custom module for OpenERP 7 that will track hardware installed in various venues. I have the total stock levels recorded in the Warehouse module, but I want to be able to calculate and display amount of each product that is available and the amount that is currently deployed. I'm having trouble figuring out how to do this. I have been looking at this rent module and they seem to do something with stock picking and workflows, but I'm new to OpenERP and not really sure how that works.

The other way I was thinking of was to loop through the deployments and simply calculate the amount of each item, and use functional fields to display it, but I'm not sure if that would even work, or how to do it without hard-coding all the various items."