Nedarim, Chapter 4, Mishnah 2. This mishnah introduces the idea that even when a person has been forbidden by a vow from deriving benefit from his fellow, that fellow may still look after him and do him good - provided that the good is done by way of removing an obligation that rests upon him. If an obligation weighs upon the one bound by the vow, removing that obligation is not considered real benefit.
The principle: "mavriach ari me'alav":
This is the expression the Gemara uses: one who chases away a lion. A person who drives off a lion that is coming to attack his fellow gives him nothing; he merely removes from him something that is troubling or harming him. So too here: there is no giving, only the removal of an obligation that was resting upon him.
The mishnah's examples:
"Shokel es shiklo" - he pays his shekel. The one bound by the vow is a person who may not receive any benefit from his fellow, and nevertheless the fellow may pay on his behalf the half shekel that every Jew is obligated to give once a year to help pay for the offerings in the Temple. He may pay it, because this is an obligation that rests upon the one bound by the vow, and he is merely seeing to it that his obligation is fulfilled.
"U'pore'a es chovo" - he may also repay his loans for him, since here too we are dealing with an obligation that rests upon him.
"U'machzir lo es aveidaso" - he may return his lost object to him.
In the case of returning a lost object the matter is somewhat different, since here there is no giving at all: the one who returns it gives nothing of his own, but returns an item that already belongs to its owner.
In a place where a fee is paid for returning a lost object:
The mishnah clarifies that in a place where the custom is that one who returns a lost object receives payment from the one who lost it, that is, where it is customary to pay for the trouble involved in handling the return, the situation changes. In such a case the person returns the lost object to the owner, from whom he may not derive benefit, and says: "I do not want to accept the payment." It turns out that benefit is reaching a person who is forbidden to derive benefit, for ordinarily he would have had to pay for the return of the lost object, and now he is exempt from that payment - and that is genuine benefit.
Therefore the mishnah says: "tipol hana'ah l'hekdesh" - the value of this benefit must be given to the Temple treasury. The one who benefits, that is, the one who does not need to pay, does not pay the person who returned the item, but gives the money to hekdesh instead, and in this way he does not benefit directly from the person who returned his lost object to him.
In summary: in this mishnah we learned that removing an obligation from the one bound by a vow is not considered benefit, on the principle of "mavriach ari me'alav" - and from here comes the permission to pay his shekel, to repay his debt and to return his lost object to him. But in a place where it is customary to take payment for returning a lost object, the finder's waiver of his fee constitutes real benefit, and therefore that benefit must fall to hekdesh.