Hi, 

you can add a related field for product in sale line / purchase line. Look for source code example in this free module - https://apps.odoo.com/apps/modules/12.0/app_purchase_requisition_stock_available/. There might be other similar modules for Odoo 10 in the Odoo store as well.

Take into account: qty'ies fields are computed, and hence would slow down sale order / purchase order opening. Especially when there might be many lines.