Can't see Transaction subtab under Customer > Sale > Transactions

A custom role must have the Financial History permission to view the Transactions subtab on a Customer record. 

To add the permission:

1) Navigate to Setup > Users/Roles > Manage Roles.
2) Click Edit beside the custom role to modify.
3) On the Permissions tab > Lists subtab, add Financial History.
4) Set the appropriate Level of access.
5) Click Save.