How To Add An Adjustment (or Payment) To A Completed Bill
If you want to add an adjustment to a completed bill perform the following steps:
• Add and freeze the adjustment ( Adjustment - Main Information).
• Navigate to the Bill - Main page.
• Click the Reopen button to reopen the previously completed bill.
• Click the Complete button to recomplete the bill. When the bill is recompleted, the system will sweep the recently created financial transactions onto itself.
You can add a payment to a bill using an analogous approach - Add the payment, Reopen the bill, Complete the bill.
Note: 
Cannot reopen historical bills. You may only reopen an account's most recent bill because recompleting the bill causes the ending-balance to change, and we don't want this to happen to historical bills.