The law of Borer allows separating between desirable and undesirable substances on Shabbat only if three conditions are met: 1) one removes the desirable food from the undesirable substance, and not the other way around; 2) one separates by hand; 3) one separates for immediate use, and not to prepare food that will be served later on.
The Halachic authorities discuss the situation of somebody who separated food planning to eat it immediately, but then changed his mind and decided not to eat the food. Has he transgressed the violation of Borer, since ultimately his act of separation was not done for immediate use?
The Sha’ar Siyun (notes to the Shulhan Aruch by Rav Yisrael Meir Kagan of Radin, 1839-199) cites (in 319:5) the Peri Megadim (Rav Yosef Teomim, 1727-1792) as commenting that in such a case one might, indeed, transgress a Rabbinic prohibition. Since he ended up separating not for immediate use, he has failed to meet one of the conditions required for allowing Borer, and has thus violated Halacha.
However, the Sha’ar Siyun draws proof from a ruling of the Shulhan Aruch (316:6) that one does not violate the Borer prohibition in this case. The Shulhan Aruch addresses the situation of one who on Shabbat stands in front of the open doorway of a house, thereby trapping a deer that had entered the house, in violation of the Shabbat prohibition of trapping. If, the Shulhan Aruch adds, somebody would come and stand near that person by the doorway, and the first person then leaves, such that the second individual now blocks the doorway, that second person does not violate the prohibition of trapping. When he first came to the doorway, he did not transgress any prohibition, because the animal was already trapped, and even now that the first person left, we do not retrospectively consider the second individual as having trapped the animal, since his initial arrival at the doorway was entirely permissible. The Sha’ar Siyun applies this ruling to the case of Borer. At the time when the person separated the food, he acted permissibly, since he truly intended to eat the food immediately. And thus when he changes his mind afterward, we will not then retroactively reassess his act of separating and say that he separated in forbidden fashion – just as in the case of the person by the doorway who initially stood there permissibly.
Sephardic Chief Rabbi Shelomo Amar adds another proof to this position, comparing this situation to the case of one who washes Netilat Yadayim with the intention to eat bread, and then decides that he does not want to eat bread. Even though his washing is now retroactively deemed unnecessary, he is not considered to have recited a Beracha Le’batala (blessing in vain) over his washing. Since at the time the Beracha was warranted – because he sincerely intended to eat bread – it is considered a legitimate Beracha even when circumstances change and the washing turns out to have been unnecessary. Similarly, since at the time when one separated the food this was done for immediate use, no prohibition is violated retroactively if the person decides not to eat the food. (It should also be noted that even the Peri Megadim, who ruled stringently in this regard, formulated his view with some degree of equivocation, stating that it would be improper to change one’s mind and not eat the food after separating, rather than saying outright that this is forbidden.)
Rav Amar concludes that it is preferable to be stringent in this regard and make a point of eating the food after separating it even if one no longer desires the food, writing about such a person, "Tabo Alav Beracha" – "He is deserving of blessing." According to the strict Halacha, however, one may certainly be lenient, since at the time he performed the act he had truly intended to eat the food immediately.
Summary: Separating desirable food from undesirable food on Shabbat is permitted if this is done for immediate use. If one separates with the intention of eating the food immediately, but then changes his mind and decides he does not want to eat the food at that point, he does not have to eat the food, since at the time of the separation his intention was to eat it immediately.
