Problem
You need to delete exported payments on an exported invoice.
Solution
Search for and go to the invoice.
On the invoice screen, unapply the payment if you need to make it a credit.
Click Add an adjustment invoice.
On the adjustment invoice screen, add a placeholder task and assign an amount equal to the credit you want to remove.
Go back to the original invoice and click Collect and apply payment.
On the Collect and Apply Payments screen, select the credit you want to remove.
Apply it to the adjustment invoice you created in step 3.
When finished, click Save.
Go back to the adjustment invoice and add a negative payment equal to the credit you are removing. For example, to remove $100.00, enter "-100.00."
When finished, click Save.
Remove the placeholder task from the adjustment invoice and click Save again.