QUESTION: The Gemara cites the Beraisa which discusses the case of a person who buys a barrel of wine from Kusim. The person has no containers available with which to separate Terumah and Ma'aser from the wine, and yet he needs to drink from the wine right away. Rebbi Meir says that he may announce that the wine that he will separate later, when he will have other containers, shall become Terumah now.
Rebbi Yehudah, Rebbi Yosi, and Rebbi Shimon disagree and maintain that one cannot separate Terumah and Ma'aser now without physically removing it, and the wine remains forbidden to drink. This is because of the rule, "Ein Bereirah" -- the status of an object cannot be designated retroactively, based on an event that will happen in the future (in this case, the consumption of the rest of the wine, leaving enough for Terumah and Ma'aser).
The simple understanding of their view is that the person's declaration does not take effect at all, and the wine remains Tevel.
However, RASHI (DH Rebbi Yehudah) writes that according to these Tana'im who rule "Ein Bereirah," the wine in the barrel is not Tevel, but rather it is a mixture of Terumah, Ma'aser, and Chulin wine. The owner's declaration does take effect to designate some of the wine as Terumah and Ma'aser, but that wine remains unidentifiable in the barrel, and thus the entire barrel is forbidden.
Rashi's words are difficult to understand. If Bereirah does not work, then it is because it is not possible to effect a change of status upon an object when that object's identity is presently in doubt. When there remains a doubt, the change of status that the person is trying to effect (in this case, making some of the wine Terumah and Ma'aser) should not be accomplished at all. The barrel should contain only Tevel!
ANSWER: Rashi's explanation is consistent with his understanding of the concept of Bereirah as expressed elsewhere. For example, in Chulin (14a, DH Osrin), when Rashi explains the view of Rebbi Yehudah that one may not drink the wine now when he announced that he will separate a certain amount as Terumah later, he writes that Rebbi Yehudah is concerned "that the person will be drinking wine of Terumah." Rashi does not say that the person will be drinking wine of Tevel, but rather wine of Terumah. This is also the way Rashi explains in Gitin (24b, DH l'Eizo, and 73b, DH u'Meshani).
It is clear that Rashi maintains that even if the Halachah is "Ein Bereirah," the change of status still takes effect, but the details which depend on the outcome of the future event remain in doubt. For example, when a person has two fruits of Tevel and says, "If it rains tomorrow, the one on the left will be Terumah for the one on the right, but if it does not rain tomorrow, the one on the right will be Terumah for the one on the left," according to Rashi one of the two fruits certainly becomes Terumah, but which one it is remains in doubt. According to the way most Rishonim understand "Ein Bereirah," neither fruit becomes Terumah because "Ein Bereirah" dictates that a present status cannot take effect based on a future event. According to Rashi, however, one of the fruits is Terumah, but exactly which fruit is Terumah remains in doubt. The change of status is effected, but the details that depended on the future event remain in doubt even after the future event occurs. The reason for this is as follows.
Rashi understands that when one makes a Kinyan dependent on a future event, the Kinyan does not depend on what actually happens in the future (since the Kinyan must take place now). Rather, it depends on what is destined -- at this point in time -- to happen in the future. Even if it actually does rain tomorrow (in the example above), those who rule "Ein Bereirah" maintain that the Kinyan does not take effect because it was impossible to know on the previous day, when the condition was stipulated, that it was destined to rain the next day. (TOSFOS to Eruvin 37b, DH Ela, cites a similar ruling in the name of MAHARI.)
TOSFOS (Eruvin 37b, DH Ela; see also Tosfos to Gitin 73b, DH Tana) and most Rishonim explain that "Ein Bereirah" means that one cannot effect a Kinyan, or a change of status, at the present moment based on the outcome of a future event. A Kinyan must take effect completely at the time that the Ma'aseh Kinyan is performed. If one attempts to make a change of status dependent upon a future event, it will not take effect at all (see RAN, Nedarim 45b). This is in contrast to the view of Rashi, who maintains that the change does take effect but it is unknown in which way it takes effect.
This also seems to be the opinion of the RAMBAM (Hilchos Ma'aser 7:1). The Rambam quotes the case mentioned in the Gemara here in which one announces that he will separate a certain amount of wine as Terumah later. The Rambam rules that one may not drink the wine after such an announcement; he must separate Terumah properly and then he may drink the wine. The fact that the Rambam says that one must perform a second act of separating Terumah and then may drink the wine indicates that the wine does not become Safek Terumah after the first announcement, but rather it remains Tevel and can be rectified with a proper act of Hafrashas Terumah later.
(See CHIDUSHEI REBBI AKIVA EIGER in Ma'arachah #4 to Eruvin 38a, DH v'Nir'eh d'Vein. See also Insights to Eruvin 37:1, Yevamos 67:1, Gitin 25:1, Gitin 73:2, Zevachim 3:1, and Chulin 14:3.)