Hi,
It's not that difficult, you just have to multilpy the tax amount by the tax rate to get back the untaxed amount corresponding to the tax amount. So if you edit the file "invoice" of the view "order" via the menu Display>Views, it should be quite easy to do if you know a bit of PHP.
For example, instead of :
<?php echo $this->currencyHelper->format($tax->tax_amount,$this->order->order_currency_id); ?>
you could use :
over <?php echo $this->currencyHelper->format($tax->tax_amount*100/$tax->tax_rate,$this->order->order_currency_id); ?> is
<?php echo $this->currencyHelper->format($tax->tax_amount,$this->order->order_currency_id); ?>