Nedarim, Chapter 4, Mishnah 7. This mishnah addresses the question of how one may benefit another person indirectly when that person is forbidden by a vow from deriving benefit from him.
The case in the mishnah:
Someone who vowed not to derive benefit from his friend and has nothing to eat is in a difficult position: his friend wants to give him food, but cannot do so because of the vow. The solution the mishnah presents: the friend goes to the storekeeper and says to him, "ish peloni mudar mimeni hana'ah, v'eini yodea mah la'asos" - "So-and-so is under a vow not to derive benefit from me, and I do not know what to do." In this way he hints to him about the problem and about the fact that he himself cannot give him anything to eat. The storekeeper gives him food, and afterward comes and collects what is owed to him from the friend. Even though this is an indirect benefit, it is permitted.
Extending the ruling to other kinds of work:
This ruling is not limited to food alone. Suppose Shimon is forbidden to derive benefit from Reuven, and Reuven wants to help him with the work that lies before him:
A house to build.
A fence to repair.
A field to harvest.
Reuven turns to the workers and says to them, "Shimon mudar mimeni hana'ah, v'eini yodea mah la'asos bishvilo" - "Shimon is under a vow not to derive benefit from me, and I do not know what to do for him." The workers go and help Shimon, and then come and collect the wages for their services from Reuven, and this is permitted to them.
In summary: In this mishnah we learned that although the one bound by the vow may not derive benefit from his friend, the way remains open to benefit him indirectly, whether with food, through the storekeeper, or with labor, through the workers. The third party is the one who gives and does the work, the friend pays him what is owed, and the indirect benefit is not forbidden on account of the vow.