ANSWERED REBBI AKIVA EIGER [as explained by Rav Boruch Ber]: The hava amina of the gemara doesn't make sense. How can we assume that since יאוש שלא מדעת הוי יאוש that means that כלך אצל יפות is also effective. Does the gemara not know that terumah requires shlichus as all kinyanim do, and that it must be a zchus to be effective [even before the pasuk אתם - גם אתם לרבות שלוחכם]? So how can the הפרשה be effective without shlichus even if יאוש שלא מדעת הוי יאוש??
So we have to say that in the hava amina we already understood that the גילוי דעת after the fact is sufficient to render it a zchus and thus the הפרשה is effective מדין זכייה
This is the proof of the gemara that יאוש שלא מדעת has the halachic status of daas from the fact that a גילוי דעת is effective in creating the reality [after the fact] that the הפרשת תרומה was a zchus.
The gemara then rejected this hava amina and said והא אתם גם אתם מה אתם לדעתכם אף שלוחכם לדעתכם - explains the Ramban, that the Torah is revealing to us that shlichus in the whole Torah requires the דעת of the משלח and זכייה that is derived from shlichus ALSO requires that the zchus should be known at the time of the action and not later.
That is how the Ramban proves from our sugya that גילוי דעת is effective with respect to terumah because the תרצן only argues with the מקשן as to the question of whether the גילוי דעת must be in advance [תרצן] or even later [מקשן] but agrees with the מקשן that in principle גילוי דעת is sufficient. For the gemara never retracted it's original assumption that גילוי דעת is effective in creating a זכות.
Of course the achronim had a field day with this terutz.....
