Here are two questions on the last whack at the kal v'chomer the gemara gives on 3b... that a person's own admission is not subject to opposition or hazama.

There is an understanding that when one obligates themselves to pay, that it is a self-obligation to giving a gift.

Before those 2 questions, I have a warm-up question -- whether such a self-obligation is akin to a public museum's required payment of a suggested donation, or a restaurant's (or caterer's) computation of their own gratuity, whether it is possible for a person to become obligated to a free will gift by another person.

But having moved forward from that warm-up question, here are two scenarios which question the framework of the gemara's claim.

1) a person says that they took some money from another person's desk, for example a student sat at an office desk in the yeshiva to learn by telephone, and thinks they took the money that was sitting on the desk. And they obligated themselves to repay.

What if the timing that the money disappeared can be proven to be after the student left for an off-Shabbos? Suppose 2 witnesses come and prove that the Baal Din was not present at the time he claims to have taken it, would that be close enough to an analogy to hazama?

(reference our coming parsha when the brothers come to repay the money that was left in their bags, but Pharoah's servant proves that Pharoah had already been paid!)

Perhaps the Baal Din's memory will be jogged and he will say, that's right, I was thinking about the money on my desk at home that I picked up. It was my own money that I put in my pocket! (no -- I'm not making an analogy to the previous sugya about grabbing his own bar of metal).

By 2 zoma witnesses if they retract because their memory has been jogged, I don't know what would be the rule. But suppose this fellow is convinced that he was guilty of the missing money from the yeshiva office desk and 2 witnesses do prove that he couldn't have been there at the time, and he doesn't retract.

2) What if I am walking by the yeshiva just before Shabbos in a windy rain storm and what appears to be a $100 bill blows onto my foot. I put it on my yeshiva office desk and get ready for Shabbos.

And what if a student takes that presumtively good $100 bill and goes to the store to buy something, but it turns out to be counterfeit! And that this can be proven by witnesses.

We now know that what he took from the office was not worth $100, but was not even a good bottle cork.

Is his testimony that he took the $100 now opposed by the testimony of witnesses?

Chaim Chesler