There is no way in NetSuite to show the credit for an existing invoice and a new invoice line on an invoice together
As an alternate solution, follow the following steps:
1. Navigate to Transactions -> Sales -> Create Invoice.
2. Enter two lines on one invoice: one to show the incorrect amount and the other one to show the correct amount using the same item.
3. Click Save.
4. Click on the Credit button from the invoice to create the credit memo.
5. On the credit memo, remove the line with the incorrect amount.
6. Under the Apply subtab, choose the invoice and change the amount to the amount of the credit memo.
7. Save the credit memo.
Note: Once you apply the credit memo to the first line (the amount that needs to be reversed / credited), total due on the invoice will change to reflect the correct amount only.
8. Go back to the invoice record and examine the amount due. It is reduced by the amount of the credit memo
Note: Once you print the invoice, it still show the total amount, however you may print the credit memo and send it with the invoice to the customer. The history of the invoice will show the credit memo and its application. The GL will be correct. You may refer under Memo field of the credit memo the invoice line that it has been created for.