Nedarim, Chapter Six, Mishnah Seven
This mishnah deals with a situation in which Shimon is forbidden by a vow from receiving benefit from Reuven, yet is in need of financial help and Reuven wants to help him. The mishnah provides legal fictions by which Shimon may derive benefit from Reuven without actually transgressing his vow.
1) If one is forbidden by vow to benefit from his neighbor, and he has nothing to eat, he [the neighbor] can go to the shopkeeper and say, So-and-so is forbidden by vow to benefit from me, and I do not know what to do. The shopkeeper may then provide for him, and come and receive payment from him [the neighbor].
2) If he had to build his house, or his fence to set up, or his field to harvest, he [the neighbor] may go to laborers, and say, So-and-so is forbidden by vow to benefit from me, and I do not know what to do. They may then work for him and come and receive wages from him [the neighbor].
Section one: Shimon, who may not benefit at all from Reuven, has nothing to eat, and Reuven wishes to help him. Reuven cannot, of course, give directly to Shimon, but he may go to a shopkeeper and tell him that Shimon has nothing to eat, and that Reuven would like to do something. Reuven should not directly tell the shopkeeper to give food to Shimon, but rather should hint at it. The shopkeeper may then provide Shimon with food and receive payment from Reuven, without Shimon breaking his vow. The crucial factor here is that Reuven did not actually tell the shopkeeper to feed Shimon.
Section two: In this case, Shimon had to (re)build his house, set up a fence, or harvest his field, but did not have any money to pay workers. Again, Reuven wants to help him (Reuven is quite a generous guy), but cannot do so directly because of the vow. As he did with the shopkeeper in the previous section, he may hint to laborers that Shimon needs work done for him, and that he would like to help Shimon, but doesnt know what to do. The workers may then go to Shimon and afterwards collect their wages from Reuven.
The mishnah needs to teach the second clause, even though the ruling should have been obvious after the first clause, in order to emphasize that this type of legal fiction is permitted even in cases not involving food. Since the case in section two is less likely to be a matter of life and death, we might have thought that in this case, the legal fiction would not be permitted. The mishnah therefore emphasizes that it is.