Due to the sales invoice being entered via FinOps, the reversal should also be done from within FinOps, as this ensures that a reversal goes into both products.
To do this:
Log into FinOps.
Browse to Selling, then Sales Invoices.
Click New.
Select the transaction type you configured for a Credit Note (usually named "Credit" or "Credit Note" but this depends on your configuration).
Enter all details as per your original invoice.
Submit and send for approval.
Once fully approved, a credit note will post into Financials. You may need to post it from the batch in Financials at this point, if you are using the batch.
The credit note will then negate your original transaction in both Financials and FinOps, rendering it "reversed out".