Is there a letter CN in front of invoice nbr? If so then likely this was created because that invoice was a debit invoice and paid by a chq. Somewhere along the line someone reverse/deleted the invoice without first deleting the payment. That causes the CN credit invoice to appear since the chq is still there. You won't be able to doucle click on CN invoices to bring it up.

If you know approximately what date that invoice would have been then pull up a general ledger report for the account payable account nbr and make sure you select to 'Show Correctons and transactions to be in 'Date' order.'. You should be able to find the original invoice with the reversal right after it and then you can figure out what had happened. Perhaps it was a duplicated posting but then the chq will need to be reversed as well in order to be able to reconcile the bank account properly. If the chq should have been reversed as well then the only way to do that is to 'pay' the CN credit invoice so that it is reversed out of bank account. You can use something like ReverseChqNbr??? as the chq nbr and date should be same date as original Chq date if still within the same fiscal year.

In my case, I deleted old invoices as part of a cleanup after I made a bit of a mess on my initial loading of customers.  Then I wanted to amend the invoice but there was "no data" when I tried to click through to it.  My fault for deleting it.

To get rid of the credit memo amount, you're going to have to balance the entry.  I've had Sage build a credit memo when a final payment was off by a buck, so I had to create an invoice with a line billing to "cash short and over" account, then a payment to both of them totalling zero.  In your case, if the credit note is nontrivial, you'll have to account for the change somewhere.

This leaves a bit of a mess in the Vendor Aged report.  We don't issue statements, so a mess there doesn't affect us.  You might have to "clear paid transactions" to get rid of it (which would also clear out transactions that you would want to show on a statement.  And be careful with clearing too much - which led to MY "no data" problem.)

I'm fairly junior with Simply/Sage.  Just tossing out a few ideas, but none are really a direct solution.

If it is a negative amount that is not a pre-payment, it likely is a credit note created by deleting or adjusting a paid invoice.  These usually show with CN after the payment (not the invoice) reference.

They can be offset against a positive invoice amount, in the Payments screen.