How to update a computed field in a rule's "set data a value" action
I cannot answer your question directly, so I will propose a solution without computed fields for your specific situation. Since your computed field is just a sum of two other fields, it is perhaps easier to replace it with an ordinary integer field Balance and update its value with a simple Rule every time the entity in question changes. In order to allow users to see the field Balance, but deny them access to edit it, you can use the Field Permissions module:


  The Field Permissions module allows site administrators to set field-level permissions to edit, view and create fields on any entity.