Hi,
Is the Original Sales Order showing the correct values still ? If yes, just try to create the Invoice with a Billing Date as 15.05.2013. It should copy the right values from the Sales Order;
If the Original Sales Order is showing the values based on from 20.05.2013 change the Pricing Date in the Sales Order to 15.05.2013 and reprice the Sales Order with Pricing Type - C[In order not to mess with any manual prices if they are present]. This should bring the Price to the same Original Value. Save the document.
Now create the Invoice as of Billing Date 15.05.2013. This should bring the correct original price.
If any of the above are not happening, there might be some other issues.
Thanks.