Hello, 

After two about 20h struggle I could not find a nice way do to that.Still I have at least a temporary solution, that solves this problem.
I would really love to get feedback from you on this. Especially how to rewrite it, so that I don't use cr and manual trims. I know this is wrong, but I could get the result in other way.

Regards,
Pavel

class stock_pack_operation(osv.osv):
    _inherit = "stock.pack.operation"
    _columns = {
                'x_sale_line_id': fields.many2one('sale.order.line', 'Custom Field: Reference to the  sale.order.line', required=True)
                }   
    def create(self, cr, uid, vals, context=None):
        tmp = [vals['picking_id']]
        cr.execute('SELECT origin FROM stock_picking WHERE stock_picking.id = %s', tmp)
        origin = cr.fetchone()
        origin = origin[0] 
        origin = int(origin[2:])
        cr.execute('SELECT id FROM sale_order_line WHERE order_id = %s AND product_id=%s', (origin, vals['product_id']))                
        zsli = cr.fetchone()    
        zsli = zsli[0]    
        vals['x_sale_line_id'] = zsli 
        res_id = super(stock_pack_operation, self).create(cr, uid, vals, context=context)
        return res_id