Gittin 26A

לא רצה האב ⁠– אינה מקודשת.

and if the father does not desire it, she is not betrothed. It is therefore evident that according to R' Shimon, there is retroactive clarification when one makes the outcome dependent on the decision of others, as in this case whether or not this betrothal takes effect is dependent on the desire of the father.

אמר ליה: בין לרבי יהודה בין לרבי שמעון; לא שנא תולה בדעת עצמו ולא שנא תולה בדעת אחרים ⁠– אית להו ברירה;

Rava said to Rav Mesharshiyya: Whether according to the opinion of R' Yehuda or whether according to the opinion of R' Shimon, it is no different if one makes the outcome dependent on his own decision, and it is no different if one makes it dependent on the decision of others; they both accept the principle of retroactive clarification.

והתם ⁠– כדקתני טעמא, אמרו לו לרבי מאיר: אי אתה מודה שמא יבקע הנוד, ונמצא זה שותה טבלים למפרע? אמר להם: לכשיבקע.

Rava explains: And there, in the Mishnah about one who purchases wine from Samaritans, they do not prohibit him from relying on the separation that he will later perform because they do not accept the principle of retroactive clarification. Rather, the reason they do not prohibit him from relying on the separation that he will later perform is as the Tosefta (Demai 8:7) teaches: The rabbis said to R' Meir: Don’t you concede that perhaps the wineskin will burst before he manages to separate the teruma, and this person will have been found retroactively to be drinking untithed produce? R' Meir said to them: When it actually bursts, there will be a concern. In any event, from the question that they addressed to R' Meir, it is clear that they would accept the principle of retroactive clarification in this case.