Yoma 56B

רבי יהודה ורבי יוסי ורבי שמעון אוסרין. אלמא אין ברירה.

R' Yehuda, R' Yosei, and R' Shimon prohibit this practice. Apparently, R' Yehuda maintains that there is no retroactive clarification.

ממאי? דילמא שאני התם כדקתני טעמא, אמרו לו לרבי מאיר: אי אתה מודה שמא יבקע הנוד ונמצא שותה טבלים למפרע? ואמר להם: לכשיבקע.

The Talmud explains the difficulty: From where do you reach this conclusion? Perhaps it is different there, as the reason is taught: The Rabbis said to R' Meir: Do you not concede that the jug might split open before he removes the portions of terumot and tithes from the mixture, causing all the wine to spill out? And he will then be found drinking untithed wine retroactively. Therefore, he cannot rely on separation that has not yet occurred. And he said to them: Although there will be a problem if it splits open, there is no cause to be concerned for this contingency in advance. Since this reasoning is based on the possibility that the jug might break, there is no proof from here that R' Yehuda rejects the principle of retroactive clarification.

אלא, מדתני איו. דתני איו, רבי יהודה אומר: אין אדם מתנה על שני דברים כאחד.

Rather, the proof that R' Yehuda does not accept the principle of retroactive clarification is from a baraita the Sage Ayo taught. As Ayo taught that R' Yehuda says: A person cannot stipulate conditions about two matters at once, e. g., one cannot establish a joining of Shabbat boundaries [eiruv] in each of two different directions on Friday afternoon while making the following stipulation: If tomorrow, on Shabbat, two rabbis arrive from two different directions, I will decide then which of the two lecturers I prefer to hear at that point in time, which will determine which eiruv is in effect.