I believe I may have discovered a logic bomb between the AR Sales Invoice and the Credit Invoice, which may also apply to the AP Vendor Invoice and Debit Invoice though I have not tested the latter.
My understanding is that, because invoice cancellation is not permitted, the Credit Invoice is intended as a full reversal of a Sales Invoice in all respects. To my logic this implies that all values entered in the original Sales Invoice should be faithfully replicated in any matching Credit Invoice.
In the upper portion of the Sales Invoice this may be the case (it is untested), but in the lower "Payment" portion of the Credit Invoice it appears that the monetary value original entered as a positive value in the Sales Invoice now has to be entered as a negative value in the Credit Invoice in order to be correctly applied in the accounts.
I have not checked to see if the quantity field for a tracked inventory item has to be similarly reversed.
All of these comments may also apply to the AP documents Vendor Invoice and Debit Invoice.
If in fact the practice of the numeric reversal by the operator is intended to be the practice then I see it as illogical hence my describing it as "logic bomb" and the correct use of the reversal invoices needs to be clearly highlighted in documentation.