Can OkiOki link invoices discounted for cash with the associated transaction?

As long as OkiOki recognizes in the invoice and the transaction the counterparty and the OGM (structured communication) or the invoice number, she will be able to link them automatically, regardless of whether the invoice amount would differ from the transaction amount by, say, 2%.

In the case of an invoice number, however, the number must be identically mentioned in the communication. So if the invoice number is "2021-06", then the notice must also be "2021-06" and not "invoice 2021-06", for example.

If OkiOki still cannot link the invoice with the transaction, you can always manually link the two yourself. You can always link a transaction to any invoice on the booking detail or vice versa.