Unconscious Contribution and Unconscious Despair in the Rambam Method

Hello Rabbi,
There is Rambam in Halachos Terumot, Chapter 4, Halachah 3:
The one who donates without permission or who goes into his friend’s field and picks fruit without permission in order to take it and donates it. If the owner of the house comes and says to him, “Go to the orchards.” If there are any orchards there, his donation is a donation, because he is not careful. If there are no orchards there, his donation is not a donation, unless he is told otherwise. And if the owner of the house comes and picks and adds, whether he has any orchards or not, his donation is a donation.
It seems difficult to see why the Rambam rules that an unwitting donation is a donation if the Gemara itself in BM 22a compares the issue of an unwitting donation to an unwitting despair, and it is known that the halakha of Abaye states that in an unwitting despair, there is no despair (and the Rambam also rules this way), and therefore I would expect that even in the matter of a donation, an unwitting donation would not be a donation. And in the Gemara there it is written, “Rava’s translation states that a messenger is not appointed,” meaning that according to Abaye, if there was no appointment of a messenger, the donation did not apply.
Best regards,