The AP Trial Balance (also called the Aged Trial Balance or ATB) lists every approved, open vendor invoice and how much remains unpaid. The total open amount should tie to your Accounts Payable GL account for the same period.

Run this report on a monthly basis and compare the total to the cumulative balance of your AP GL account. If they don't match, use the troubleshooting queries in the P21 KB to locate the imbalance — common causes include manual GL postings to AP, voided prepayment checks, and vouchers posted to a different period than their GL entry.

Each voucher's open balance is calculated as:

Open Balance = Invoice Amount
    − Amount Paid
    − Terms Discount Taken
    + Memo Amount

Payments and terms discounts are only counted if posted in or before the current period. Void checks with a void date in a future period are excluded from paid amounts.

When a voucher is marked Disputed in P21, it is excluded from check printing windows and appears in the Disputed column on this report. The disputed amount is still posted to the GL normally — only the payment eligibility is affected.

As of P21 v12.6, disputed vouchers are not included in the Total Open Amount by default. When reconciling to the GL, add the disputed balance back to get the true AP liability.

Vouchers are aged by their net due date — the date payment is due after applying the vendor's payment terms. Buckets: