The Gemara says that Rav could even work out in R’ Elazar B’r Shimon – despite the fact that ordinarily Rav holds that if one admits to the fine he won’t have to pay it, where he’s admitting because of the approaching witnesses, he will still have to pay the fine.
This needs explanation! Why should it matter what his motivation is? If the Torah says one is exempt when he admits, and as Rav says, this applies even where witnesses later arrive, what do we care why he’s admitting?
In order to understand this, we need to think about why the Torah does not require one to pay a fine if he admits he has done the act that ordinarily requires payment for the fine. The simple understanding is that the Torah uses the fine as a teaching tool to help one realize the crookedness of his ways. The greater that evil act, the greater the fine – the greater this person needs to get a ‘potch’ to realize his mistake. Thus if he only steals, he only pays double. If he goes so far as to slaughter the animal or sell it, he must pay four or five times.
The reason the Torah says that if he admits to the act he’s done he does not have to pay is because he has shown that he understands the mistake he has made. Thus, we no longer need to ‘teach him a lesson.’ (Parenthetically, this explanation works for Rav, but according to Shmuel, we will need to understand why if he has admitted, we will still give him the fine when witnesses come along without his knowledge.)
With this background, we can understand why Rav would concede that if he admits when he sees witnesses coming, he will still pay the fine. He is only exempt from the fine when we see he has learned the lesson and has admitted on his own. When he admits because the witnesses are coming, he is just trying to get out of having to pay the fine, not because he has learned his lesson! Thus we require him to pay, despite his admission, so his pocket will hurt and he will learn that crime does not pay.