In PA u can only create negative exp thru preapproved exp batches.
And coming to (create negative unmatched expenditure items so this would have produced a stand alone invoice and not a credit memo to apply to the original invoice)
Project Source in AR, select the Invoice type which has +ve sign.
We have several Transaction Sources defined in PJC to allow entering negative independent transactions. That allow us to interface either positive or negative transactions using the same source.
You could also generate a project invoice to the customer. In case all expenditures included in the invoice are negative, you would get a negative invoice sent to AR. Such invoice is not considered a debit memo. You can apply cash received from the customer in AR to positive as well to negative invoices.