Whenever we scan a receipt or interpret a user expense request to extract the details, we ask the user to confirm the details. At that point the user can fix any details captured incorrectly. We allow the user to update the expense category, amount etc.
If an expense has been confirmed by the user and sent for manager approval, but not approved by the manager yet (pending approval), it can be modified by saying "Modify" and then selecting from among all your expenses pending approval.
If an expense has already been approved by the manager, then the only way to modify it is for the manager to say "Modify @username". An approved expense cannot be modified by the employee himself/herself.