You don't pay a fare difference if your eUpgrade does not clear because in this case you have not moved into a new booking class.

Let's say you're on a Tango Plus S class fare. You request an upgrade but you are waitlisted. In this case, you still have your S class booking. You continue to wait until the flight boards, but, alas, your eUpgrade request is not successful. No harm, no foul, you still have an S class ticket.

Now, what rehoult was referring to is the case where you buy the S class Tango Plus ticket as in the example above, but your upgrade does clear. Now you are in a different booking class - R class. So you no longer have S class space but that's what you paid for. If you were to now cancel your eUpgrade request and there is still S class inventory left, you won't pay anything else. But if S class is no longer available, then you will have to pay the difference between your original S class ticket and the new fare class that is available (let's say H).

Does that make sense?