This might not going to be the most commonly used feature but it might come in handy and have its audience. Also, it should not be too hard to implement into the existing UI and backend (assuming you have full control over the backend functions involved).
Often exchange rates are volatile and can change within seconds. Particularly when exchanging larger amounts, that can turn into a substantial amount and can happen between choosing the amount and submitting the exchange request.
In order to avoid that it would be nice to be able to specify a rate window along with the amount, and only if the eventually applicable rate falls into this window the system will actually go through with the exchange and fail otherwise.
I cant say how easy that could be implemented as I am not sure if Revolut could propagate these two values (the exchange window) up to the point where the exchange takes place, but if you have control over it it might be fairly easy to do.
As I mentioned initially I do not assume that feature would be the most commonly used, so I could easily imagine to hide it behind an “Advanced” button/slide-out on the exchange dialog.