The classic case of preventing damage (מבריח ארי) is where Reuven sees a lion headed toward Shimon’s flock. Reuven chases away the lion to prevent the damage it would cause to Shimon. There is no monetary value to this action of Reuven, despite the fact that he has saved Shimon a tremendous amount.
The case under discussion in our gemara is where Reuven has a field that Shimon’s animal accidentally trespasses upon. The field is quite muddy, and Shimon’s animal is in danger of being damaged in the mud. Reuven unwittingly helps out Shimon’s animal when the animal tramples on some wheat stalks. While this trampling saves Shimon’s animal from damage, it simultaneously causes the wheat stalks to be destroyed. There is a value in this case to the damage that Reuven has unwittingly prevented. Shimon must pay Reuven for this benefit.
The Gemara says that our case is different than the aforementioned one for two possible reasons. One is that in the former case, Reuven was fully aware that he was saving Shimon from damage, which is not so in our case. The second reason is that in the former case there was no loss of money when he chased away the lion, which is not so in our case, where the prevention of damage to the animal inflicted damage on the wheat stalks.
The first distinction needs explanation, because of what significance is it if the person is aware? Just because he knows he is saving the other person’s property, would it be as if he is relinquishing his claim to monetary compensation? If we would say that the issue is that when he knows, so he accepts upon himself any damages, then what is the difference between the first and second answers? The gemara seems to indicate that they are independant (from the language אי נמי)!
A careful reading of Tosfos answers this question.
Tosfos explains that the first answer is assuming that מבריח ארי is only when the savior is aware he is preventing damage, and thus he implicitly relinquishes his claim to monetary compensation, even if he loses money on the way. In our case, however, he is not aware, and thus there is no implicit relinquishing of the compensatin for loss.
The second explanation of the Gemara assumes that when a person is not intentionally preventing damage, it would also be called מבריח ארי. Thus, Reuven would only be left without compensation in a case where there is no damage to himself. If he is unaware, and there is indeed damage, so there would be an obligation to pay the damage, as in the case of our gemara.
So it comes out that the two answers are simultaneously independent and interdependent; the main focus of the first answer is the aspect of awareness, and the main focus of the second answer is the damage, but both include the other factor as well, just in a different light.