To correct a client's balance you can go to Customers / Customers, and look for the customer on the list.

To access the customer's file press  

Once there go to "Billing and Collection"  on the left menu and to "Balance" tab after that.

Click on  "Balance Correction" button and you'll see the following window.

Type in the total amount the customer should have on credit, and the a description if you like to, and click accept.

If you want to know how to share a customer's credit with another customer see how to do it here --> Share credit/balance between customers