To Compute the quantity invoiced If case of a refund in openerp to Compute the quantity invoiced. If case of a refund, the quantity invoiced is decreased. Note that this is the case only if the refund is generated from and function given below to use this code in .py file.

 @api.depends('invoice_lines.invoice_id.state', 'invoice_lines.quantity')
    def _get_invoice_qty(self):
        for line in self:
            qty_invoiced = 0.0
            for invoice_line in line.invoice_lines:
                if invoice_line.invoice_id.state != 'cancel':
                    if invoice_line.invoice_id.type == 'out_invoice':
                        qty_invoiced += invoice_line.quantity
                    elif invoice_line.invoice_id.type == 'out_refund':
                        qty_invoiced -= invoice_line.quantity
            line.qty_invoiced = qty_invoiced