When posting the actual disbursement either PIO/BPO/ND as long as the correct AD is picked up and incurred then if the AD is billed the disbursement will be marked as billed.
If the AD is not billed then the disbursement posting will clear the outstanding AD and only leave the PIO/BPO/ND to be billed against.
If the full amount of the AD is not picked up to be paid but the full amount is billed and the bill paid this will cause a credit balance.
The way to correct this is to either do a minus AD for the half not needed or to pay the AD balance using ND/BPO postings.