The transaction detail line is posted. So you are unable to delete the transaction.The Invoice needs to be posted completely and then you will need to post a credit note to reverse it. As the Detail line is posted you cannot delete the transaction and cannot unflag it in the backend either, because the transaction is 50% completed and waiting for the other 50% to be completed.
Once you post this invoice, then the transaction will be completed. And then you can post a credit note to adjust the balance.
