Over charged sale refund

Hi,

We have invoiced an amount for a job as per the purchase order and this amount was paid.

They have now come back and said the purchase order amount was incorrect which we argree on.

We need to credit the invoice to deduct the incorrect amount (which I would normally credit then apply to invoice)  but I also need to refund the extra.

I can't change the original invoice as it is in a locked quarter.