Starting with version 8.10.0, you can now correct a payment that has been received for an invoice in Atrium without having to contact LSP Ware.  It is important to note that this reversal does not transfer to Quickbooks.  You will need to delete the payment received for the invoice manually in Quickbooks.

You start by locating the invoice that the payment was received for in Financials/Customer Invoices:

Note the reverse arrow icon to the right of the envelope.  You need only click that icon and confirm when the warning appears:

You will receive confirmation:

and that invoice will now show as unpaid in Atrium.