In our past couple of posts (here and here), we have discussed the notion of a petach, an opening to allow one to annul a vow. The operating premise is that the vow was valid; however, upon being made aware of an aspect that had he been cognizant of such when making the vow he would not have done so, we can annul such a vow.
  
There are two other types of "potential annulments" discussed in the 9thchapter of Nedarim, nolad and taut. Nolad, literally "born", refers to a major new development occurring after a vow is taken which, had it occurred earlier, would have ensured that the vow would not have been taken. Rabbi Eliezer is of the view that this new development is grounds for annulment (Nedarim 64a).
  
In the world of business, such a claim would be ludicrous. Purchasing a house just before interest rates take a sharp hike upwards does not give one the right to renege on the deal (even before the actual closing). The nature of business is that people have different expectations of what the future will bring[1]. If not for this difference of opinion, the stock market would cease to exist and much business would grind to a halt--making us all poorer. One cannot revisit the past based on what happens in the future. And in fact, we accept the view of the Sages that nolad is not grounds for annulment.
  
Rabbi Eliezer, however, disagrees, distinguishing between the world of economics and that of promises made. "And Rabbi Eliezer further said: We allow an opening [to annul a vow] based on nolad. How so? He said, konam, forbidden that I should benefit from that person, and he became a sofer, scribe, or he is marrying off his son and he says if I would have known that he would become a sofer or would soon be marrying off his son I would not have vowed...Rabbi Eliezer permits [such an argument] and the Sages forbid" (Nedarim 64a). Unlike in a market model where there is a buyer and a seller with differing expectations[2] with a vow there is no other party who "loses" if a  vow is annulled. New events always cause a re-assessment of our actions - regarding a vow such re-assessment allows one to nullify a pre-existing vow.
  
The opposition of the Sages to this form of retroactive annulment is such that even when one clearly lays out the reason for the vow they may still not allow for nullification if those conditions do change. "How so? He said konam, it is forbidden if I marry this woman because her father is evil, they said to him he has died or he has repented" (Nedarim 65a). Despite the fact that it is clear that he did not want to marry this woman because he did not want to have a wicked father-in-law he did not actually make his vow conditional on the wicked father in-law. Thus even though he will in fact not have a wicked father-in-law the Sages argue that he still may not marry this woman[3], considering this to be a case of nolad. Rabbi Meir disagrees and considers his reasoning to be a form of a conditional vow. 
  
Interestingly, in discussing the Mishna Rav Yochanan explains that we are dealing with a case not where the father in law subsequently dies (or repents) but rather unbeknownst to the "groom" the father in law had already done teshuva or died before the vow was actually taken. Thus Rav Meir says it is akin to nolad but it is not actually nolad as the change had occurred before the vow actually occurred and the vow can be annulled (or is automatically annulled - see Tosafot s.v. Rav Yochanan) on that basis[4].
  
What is fascinating is how Rav Yochanan would explain the view of the Sages - "and they did not agree with him". The simple explanation is that despite the fact that he had already repented, nonetheless the Sages say this vow cannot be annulled. But why? The Tosafot (s.v. Rav Yochanan) explain that this is actually a rabbinic enactment lest one annul vows when no repentance has been done. With repentance not so common the rabbis were worried that if one allowed annulment in this case, one would mistakenly annul a vow even if no repentance had occurred (or he had not died). 

The Rosh (s.v ukvar met) however gives the exact opposite interpretation - with the Sages reflecting the lenient view. "And they did not agree with him" means not that we can't annul such a vow but rather there is no need to annul the vow - as this is a vow made in error. And all agree that vows made in error are ipso facto annulled automatically.
 
"Who is wise? Haroeh et hanolad, one who foresees what might occur" (Tamid 32a). Unfortunately too often we only take into account what might be when it is too late. The wise take time to think before they act and speak.
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[1] The notion that the stock market goes down because "people are selling their stocks" is of course inaccurate. Sellers can only sell if there is someone willing to buy. What they mean to say is since there are more people looking to sell stocks at a given price than there are buyers at that price, prices will be forced down until there are an equal number of buyers and sellers. (I apologize for foisting my accounting background unto these pages). 
 
[2] Ideally in a business deal the transactions benefits both. But some business transactions are a zero-sum game where my gain comes at the expense of your loss. That is generally how trading in the stock market works.    
  
[3]Rabbi Meir disagrees considering his vow a conditional one  would allow for nullification of the vow by asking had you known the woman's father would do teshuva would you have made such a vow. When he answers in the negative the vow would be annulled.
 
[4] Perhaps one can also explain that while the father-in-law had already done teshuva and thus it is not nolad, from the perspective of the groom it would be considered nolad