If Xero is enabled to write-off an already raised invoice that has been pushed through to Xero you must do so via a credit note.

From the green "+" quick create and select create credit under billing.

Next select the invoice that you wish to credit from the drop down list

The next screen will automatically pre-fill with the fees and expenses as per the original invoice is raised.  From this screen you can choose to credit the entire invoice by simply hitting save or you have the option to only credit part of an invoice by simply changing the dollar amounts OR you can even credit just a timekeeper/fee earners time by removing the other time keepers.  

If you wish to just credit the fees and continue to bill the expenses again you can simply remove the expenses from this screen and just credit the fees.

Once you have made your changes hit save at the bottom of the screen.

Now that the credit has been raised you now need to apply the credit against the actual invoice and this is what pushes through to Xero in the same way that a payment pushes through.

Again from the green "+" quick create button select apply credit under billing.

From this screen select the credit note that you have just raised from the drop down list.  NOTE the credit numbers will be CR## (## being the same as the original invoice number).  Once you have selected your credit note you complete the bottom of the screen as you would be receipting a normal payment of an invoice by selecting the ">" which pushes the dollar amount that you are paying across and hit save.

The original invoice will now be marked as finalized/paid.