During data entry, editing will be in the Home currency values and changing these will affect the foreign currency value. However, the reverse is not true unless the Option 'Currency Values' is ticked. This can be done by following the below steps:
Go into Selling, New Order.
Enter the Account and Sub-Ledger.
Click the Options button.
Check 'Currency Values' and close the options window
Ensure that when exiting Dimensions and it asks "Do you wish to save the transaction options you have defined?" that you answer Yes