In iVendNext, you can fetch Exchange Rates between currencies in real-time, or save specific exchange rates as well. In iVendNext, saved exchange rates are also referred to as Stale Exchange Rate.
In your sales and purchase transactions, if the field of Currency Exchange Rate is frozen, that is because the feature of allowing stale exchange rates in transactions is enabled.
If you wish to make the Currency Exchange Rate field editable again, then disable the feature of Stale Exchange Rate.
Navigate to: Accounting > Accounting Masters > Accounts Settings
Under the 'Currency Exchange Settings’ section uncheck the field "Allow Stale Exchange Rates".
Save Account Settings
Check Sales / Purchase transaction once again
After this setting, the Exchange Rate field in the transactions should become editable once again.