I need help setting Odoo v8 for each sales order automatically check if the products have stock in the warehouse enough to serve that request and otherwise create a PO (Purchase order) of the missing products. 

After reading hundreds of pages and forums I've only seen the ability to configure products by MTO (make to order) that make me a PO even if estoc in the warehouse. Or by MTS (make to stock) with a minimum product rule. 

The problem is that many of the products in my catalog I do not care to have them on stock, but that will asking the supplier according to realization of sales. But many of these products are left on stock me because exchanges, returns, etc. 

That is, when you enter a sales order I need the system first check if the product is in the warehouse and if there are none, request the supplier only the quantities sold. 

Can anyone help me, please?