Hello Faisal,

If you want to add fields in purchase.order.line then inherit this class purchase_order_line and add fields according your client requirment and use xpath in .xml file in your module .
Exp- In .py file in your module

class purchase_order_line(osv.osv):
    _name='purchase.order.line
    _inherit='purchase.order.line'
    _columns={
            'cartage':fields.float('Cartage',readonly=False),
                }

And then add same fields account.invoice.line to inherit account_invoice_line class form account module and also inherit the this function given below.

class account_invoice_line(osv.osv):
    _name='account.invoice.line'
    _inherit='account.invoice.line'
    _columns={
            'cartage':fields.float('Cartage',readonly=False),
                }

def _amount_line(self, cr, uid, ids, prop, unknow_none, unknow_dict):
        print"==========_amount_line222=========="
        res = {}
        tax_obj = self.pool.get('account.tax')
        cur_obj = self.pool.get('res.currency')
        for line in self.browse(cr, uid, ids):
            price = line.price_unit * (1-(line.discount or 0.0)/100.0)
            taxes = tax_obj.compute_all(cr, uid, line.invoice_line_tax_id, price, line.quantity, product=line.product_id, partner=line.invoice_id.partner_id)
            res[line.id] = taxes['total'] + (line.quantity * line.cartage)
            print"==========Add here fields cartage ==========",res[line.id]
            if line.invoice_id:
                cur = line.invoice_id.currency_id
                res[line.id] = cur_obj.round(cr, uid, cur, res[line.id])
        return res

If you want to Get custom field in invoice from purchase order in OpenERP (Odoo) follow the below mentioned steps:

Step-1 Go to settings Menu and click it.
Step-2 Go to sales -> sales form content -> custom fields-> PURCHASE ORDER.LINE all three available fields > After that click on done Button

In Odoo/OpenERP-8 we can inherit product.template model or inherit the tree view of purchase.order.line and add the invoice filed .( by inheriting and using xpath in .xml file)

For Example..

field_name = field.Char(related="product_id.invoce_line", string="add_newfield") ,

yeah i want that in odoo v8 i inherited that field using xpath in purchase.order.line but i want that field also in invoice and i want to calculate totat and subtotal amount based on that field if price_unit field is empty or equal to 0