An invoice that has gone out is cancelled, not deleted

Once an invoice has been sent, the Delete button will refuse
you. Cancel it instead.

The mistake

You spot an error on an invoice the client already has. You try
to delete it and start again. LEAT refuses.

Why

The client is holding a piece of paper with a number on it. If
we delete our copy, the client's copy is the only record that
the claim was ever made, and our books and their books now
disagree with nothing to explain the gap.

Cancelling leaves both. Ours says cancelled. Theirs still says
what it said. Anybody can see what happened and in what order.

What to do

1. Open the invoice and set its status to Cancelled.
2. Raise a new invoice with the right numbers.
3. Tell the client which number replaces which. Do not assume they will work it out.

What you can still delete

A draft nobody outside LEAT has ever seen. If it has never been
sent, it is a piece of work in progress and deleting it costs
nothing.

Even then, the record of the draft's existence stays in the
audit journal. That is deliberate: the journal is append-only,
and it does not go away with the row it describes.

Payments are the same

A payment recorded in error is reversed, not deleted. Both the
original and the reversal stay visible, and the reversal
carries the reason.