The Related Records tab is not a default permission to some roles/customer roles, this privilege needs to be added on the role so that all transaction records/history of a record being viewed using several roles will show the 'Related Records' tab , like the one that an administrator can access. 

To set that, using administrator role, go to : 

1) Setup>User/Roles>Manage Roles 
2) Select the custom accountant role>Edit 
3) Permissions>List
4) Add : Financial History

5) Set the *access level of permission accordingly 
6) Save

Using the modified custom role, when you navigate to a transaction/record, the related records tab will be available. 

*Access Levels for Permissions

The following are general definitions of possible access levels for permissions.

• VIEW - User has access to view existing files only. The user cannot create new, edit existing, or delete existing files.

• CREATE - User can create new and view existing files. The user cannot edit or delete existing files.

• EDIT - User has access to create new, view existing, and edit existing files. The user cannot delete existing files.

• FULL - User has access to create new files and view, edit, and delete existing files.

Note that for some permissions, only the minimum view level is required for usage, and other levels do not provide any additional capabilities. In the model defined by the above access level definitions, each successive level (view, create, edit, full) of a permission provides increased usage of the related record type, task, or page, but the usage of some permissions does not fit exactly into this model. Generally, any user with at least VIEW access to a record type has the ability to print records of that type.