Moayad, On Hand Available quantity is a calculated field.  To change the quantity on hand, you need to create stock.move.  Inventory Stock model is a way to allow you to create a stock.move that makes the quantity to be the figure you specify.  The stock_change_product_qty is the wizard that you can use to create Inventory Stock (see the method change_product_qty of that model) for a particular product.

So, if you have a couple of fields added to specify the quantity on hand in the product.product model, one way you can actually change the quantity on hand is by triggering stock_change_product_qty (update link) and set the new_quantity field of that model to be the figure that you want.  To do so, you can explore the default_get method of ORM.