Invoices with Negative Total Amount

[Nestor Stura]

Hi all,
First of all my apologies because maybe this could be a very simple doubt,
but, as I am my own "CPA" and I know nothing at all in accounting, I must
post this.
I also posted it in the Spanish list but had no answer.

In Spain we cannot delete an invoice, then when you should return a previous
invoice we must issue a new one called "factura de abono" (something like
"refund invoice") with the same amount of the first one.

The problem is that in Gnucash when I finish this refund invoice and try to
post it, I get a message that Negative Total Amounts are not Accepted.

It is not a problem with the unit quantity or the unit price that are
accepted. The problem is with the total.

Does anyone know if there is a way to do this?

Thanks in advance,

Néstor