I am somewhat baffled on the concept of when an aveirah occurs. Let's take 2 examples:
1. As of today, King David did not commit adultery with Bathsheba because once Uriah died she was divorced retroactively and therefore single at the time of King David. This works when we look at it today. But at the exact time that they were having relations, Uriah was alive. Thus, it seems AT THAT POINT that King David sinned. How does one sin and then that sin go away at a later date due to another incident?
2. In our gemara the man hears of the boy in the water but casts his net only for fish. If, when pulling in his net, he catches 4 fish and then seconds later catches the boy, Rabbah says he is not liable as he caught the boy. Again, that works from the perspective of once the net was reeled in. But at the moment in which he caught 4 fish and no boy, it seems he does the melachah of trapping. Again, how does a sin occur and then it is wiped out due to another occurrence?
Barry Epstein, Dallas, USA
This question and a few more were lost on a different computer and I never had a chance to see them and address them. However, Torah is timeless, and so I am responding now.
Allow me to expand your question somewhat. You asked about time and I will relate to time and intent. The Gemara says that one who intended to eat pork and accidentally ate lamb needs atonement. The Gemara cites a verse to prove this. Although the person needs an atonement, it is not for eating pork. Just as the Mitzvos have exacting standards, and one who shakes a lemon instead of an Esrog with all the good intentions in the world does not fulfill the Mitzvah, so, too, an Aveirah has exacting standards, and although this person needs atonement for his willingness to transgress the will of Hash-m, he still did not eat pork.
With that introduction, let us relate to your question. The case of the fisherman is slightly more involved so I will deal first with David. Let us look at a case of a Safek. For example, a man dies childless, leaving two wives. One is pregnant. If her child is carried to term and lives, then there is no Mitzvah of Yibum, and both women are forbidden to the deceased husband's brother with the Isur of Eshes Ach. If her child does not survive, then there is a Mitzvah of Yibum. If the brother does not wait (as he should) to see whether the child is born and lives and he goes ahead and marries one of the wives, then if the baby dies he has not transgressed, but if the baby lives then he has committed a very severe sin. This is because the eventuality is not yet known and the act of the sin depends on whether or not there will be an obligation of Yibum.
A slightly different case is that of the two-loaf Shevu'ah. A person swears that he will not eat loaf A if he eats loaf B. He then eats loaf A. He has not done any Isur per se. Rather, he has done a contingent Isur. If he does not eat loaf B, then he is fine. But if he does eat loaf B, then it becomes evident that his eating of loaf A was a sin, a transgression of his oath. There are situations in which something is not clear (or contingent) at the time of the act, and we do not know whether it is an Aveirah or not.
Whether or not one is allowed to take the chance is a different matter. However, if one does take the chance, still he cannot transgress the Aveirah in question if, for example, the meat that he eats is not actually pork.
Parenthetically, it seems clear from the sources that David ha'Melech's act did involve some minor degree of misdeed. The Gemara says that he was a pioneer of Teshuvah, and he himself says countless times in Tehilim that he sinned. However, he did not commit adultery, for it became evident that she was not actually a married woman at the time.