As Admin, you can always correct Cutomer's balance.
Here are the steps:
- Open "Manage customers" page in Admin backend (yoursitename /exec/manager/customers)
- In column balance, find a "+" button near customer's balance and press it. You'll be forwarded to choosed customer's transactions.
- Find a correction block in the end of the page.
- Specify needed amount and left a comment for client
- You can also set a negative amount to remove credits from customers account.