The RadBusyIndicator doesn't support such feature - you cannot let some buttons inside the RadBusyIndicator to stay enabled as it just makes its content disabled. What I would suggest you is to either place this cancel button outside the BusyIndicator or to place another cancel button in the BusyIndicator overlay. This can be achieved by changing the ControlTemplate of the RadBusyIndicator control.
Hope this helps.
the Telerik team
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