What is the machlokes אביי ורבא by יאוש שלא מדעת? (Bava Metzia 21b-22b)

The פשוט פשט would be that רבא holds that his יאוש at the end is מגלה that he was really מתייאש all along. However, this is very difficult as how does that work? In fact, the ראב"ד asks this should be תלוי on the מחלוקת whether or not יש ברירה.

Another way of explaining the מחלוקת is as follows. אביי holds that you need יאוש בפועל, the person has to actually be מתייאש. The fact that we know the person will be מתייאש when he finds out is irrelevant, until he is actually מתייאש it is still his. רבא, on the other hand, holds that since it is עומד ליאוש, the average person would be מתייאש in this situation , the Torah permitted you to take it even though there was no יאוש בפועל. In fact, the Ritva writes that רבא holds that יאוש שלא מדעת works even if the person is never actually מתייאש which fits in well, it is not dependent on him, rather the Torah was מתיר where it is עומד ליאוש.

We see from תוס' ד"ה דלאו בני מחילה (on 22B), that they hold like this as well. Tosafos writes that according to רבא there is no question from קטנים because when they grow up they are מתייאש. We can't understand the תוס' כפשוטו that the יאוש at the end is מגלה that they were מתייאש all along, because they were קטנים and could not be מתייאש. Rather the פשט has to be as we explained that רבא holds that since it is עומד ליאוש the Torah was מתיר and therefore the fact that they were קטנים at the time is irrelevant, because it is not dependent on their יאוש, rather the Torah was מתיר.

הכא במאי עסקינן כשיכול להציל (Bava Metzia 22A)

There are 2 different מהלכים in the Rishonim how to explain what the Gemara means here.

1. The way it was explained in shiur which is Rashi's pshat is as follows. The ברייתא is difficult on רבא because the דיוק is that if he is not מתייאש the finder can't keep it which implies that יאוש שלא מדעת לא הוי יאוש. The Gemara answers that רבא has a different דיוק (and consequently a different pshat in the ברייתא). According to רבא the דיוק is that if he can save it (כשיכול להציל) then we don't say יאוש שלא מדעת הוי יאוש because he is not מתייאש. According to this it comes out when the ברייתא says מפני שנתיאשו הבעלים it means יאוש שלא מדעת. The ברייתא said it that way so that we could be מדייק that in a case of יכול להציל there is no יאוש שלא מדעת. As was explained in the shiur the הכא במאי עסקינן is going on the דיוק not the case.

2. Most of the other Rishonim reject Rashi's pshat (as it is very difficult in terms of the language of the Gemara) and explain as follows. הכא במאי עסקינן is changing the אוקימתא of the ברייתא. The answer for רבא is that the case of the ברייתא is a case where he can save it (כשיכול להציל) and therefore there is no דיוק fromמפני שנתיאשו הבעלים that is simply the דין because where he can save it we can't assume that he would be מתייאש. Therefore the ברייתא is not a question on רבא as there is no דיוק, it is simply telling you the דין that in a case where it is יכול להציל there is no יאוש שלא מדעת because we assume that he is not מתייאש.