The Administrator needs to give permission to the custom roles to edit the Exchange Rate on transactions but not allow them to update the Consolidated Exchange Rates page.
The permission for Currency is only set to View but the user needs to edit the Exchange Rate, which requires at least Edit level of access.
As an alternate solution, the Administrator can create a workflow to set the display type of the Exchange Rate field.
1. Navigate to Customization > Workflow > Workflows > New.
2. Provide a Name.
3. In the Record Type, select Transaction.
4. In the Sub Types, select the transaction types.
5. Set the Release Status to Released.
6. In the Event Definition, mark both On Create and On Update checkboxes.
7. Click on Save.
8. Click on New State > Save.
9. Click on State 1.
10. Click on New Action > Set Field Display Type.
11. In the Event Type, highlight Create and Edit.
12. In the Parameters > Field, select Exchange Rate and set the Display Type to Normal.
13. Click on Save.