Reversing an invalid invoice that has a ginormous line tax total

Hi,

A client currently has an invoice that has somehow been created with a ginormous line tax total, and it is causing the month to month balances to go out of whack, as shown below.

They are understandably tentative to post this invoice to the General Ledger, and have thus not posted anything for the last few days to remove the risk of accidentally posting it through.

What would you recommend is the best way to reverse an invoice like this to ensure the balances are moved back to where they should be, and so they are able to resume posting transactions to the General Ledger?

Kind regards,

Andrew