You can maintain currency exchange rate via transaction OB08, since it’s only table maintenance view which using by transaction code SM30, SAP doesn’t provide specific transaction code for display only.
In client you need to maintain OB08 ( Exchange rate ) for displaying only to some users, to solve this problem you can maintain authorization from transaction code PFCG using S_TABU_DIS object
1.Go to transaction code PFCG, create new role or edit existing role. for this sample we will create new role.
2. Click on Menu tab and add transaction code OB08.
3.Click on Authorization tab and Change Authorization Data
4.Go to menu Edit->Find, and find object S_TABU_DIS
5.Add Table Authorization Group FC32 and Activity 03 ( Display )
6. Assign this role with your desired user to apply this role.