The Shulchan Aruch (O’H siman 489:4) addresses the situation of a person who prays Arvit with a congregation that recites Sefirat Ha'omer during the period of Bein Ha'shemashot, which is earlier than the preferred time for counting the Omer. This individual prefers to count the Omer later, after dark, but fears that if he delays the counting he will likely forget to count. According to the Shulchan Aruch, a person in this situation should count with the congregation without a Beracha and stipulate that he fulfills the Mitzva with this counting only if he does not remember to count later, after dark. If he does remember to count later, then his earlier counting is null and void, and he may now count the Omer with a Beracha.
This concept, of performing a Mitzva on condition that one will not have the opportunity to perform it later, applies in other areas of Halacha, as well. For example, occasionally a person is unsure whether or not the congregation will reach the Shema recitation before the final time for Shema. He thus faces a dilemma: on the one hand, he should perhaps recite Shema ahead of time, before the prayer service begins, to ensure that he fulfills the Mitzva; but on the other hand, it is far preferable to fulfill the Mitzva of Shema through the recitation of Shema in the "Yotzer Or" section of the prayer service. Thus, a person in this situation should recite the Shema before the prayer service on condition that he fulfills the Mitzva at that point only if the congregation does not reach Shema before the final time; if the congregation does reach Shema in time, then his earlier recitation does not count towards the Mitzva.
Similarly, if a person hears Havdala in the synagogue on Motza'ei Shabbat and he is uncertain as to whether he has wine for Havdala at home, he may have in mind to fulfill his obligation through the Chazan's recitation, on condition that he has no wine at home. He should stipulate that if he does have wine at home, then he does not fulfill his obligation through the Chazan's recitation of Havdala in the synagogue, and then he may recite Havdala at home.
Finally, Chacham Ovadia applied this rule to a case of Birkat Ha'levana when a thin cloud covers the moon. Although Halacha permits reciting Birkat Ha'levana when a thin layer of cloud covers the moon, provided that the moon provides sufficient light from which one can derive benefit, some people make a point of waiting until the moon is completely unobstructed before reciting the Beracha. If a person in such a case is uncertain whether he will have the opportunity to see a perfectly clear moon and recite Birkat Ha'levana later that month, he may listen to the Chazan's recitation and have in mind this same kind of condition. He should stipulate that he fulfills his obligation at this point only if he is unable to recite the Beracha over a perfectly clear moon later in the month; if he does have such an opportunity, then he does not fulfill the obligation now, but rather when he recites the Beracha later in the month.
See Hazon Obadya, Hanukah, page 323.
