The above illustrated bug has wider issues then just the one illustrated above.
A) If the starting credit balance is refunded via a payment prior to the invoice (6,000) being created and that invoice includes a tax component and remains unpaid, then on the tax summary report - cash basis, that unpaid invoice is listed as a “paid” transaction due to its false “Paid in full” status as illustrated above.
B) The aging section on the report > Customer Statements (Unpaid invoices) is not correct.
Refer to the topic Non Cash transactions appearing on Cash Basis tax reporting - #9 by Solnce for illustrations.