Hi
Carlos
I tested the same thing in version 9 patch 6 and system allows me to cancel the document.
I think you can approach lock call with SAP for this.
Below Scenario i tested
1. AP downpayment request (amount 100)
2. Outgoing payment (amount 100)
3. AP Invoice (amount amount 60, adjusted DPM-50, AP Invoice balance due -10)
4. Cancel AP Invoice, system give same warning message while cancellation (cancellation is irreversable)
5. Create new AP Invoice (amount 110 , adjusted DPM-100, AP Invoice balance due -10)
Thanks & Regards
Unnikrishnan