I am in charge of purchasing sefarim for my yeshiva. I solicit funds from donors to cover these expenses. Someone asked me to purchase a Shas for the yeshiva. The bookstore gives the yeshiva a 20% discount since we purchase many sefarim.

Q: Am I permitted to charge the donor the regular price for the Shas and keep the difference to purchase other sefarim?

A: We must begin with an introduction concerning the halachos of shelichus (agency). If Reuven sends Shimon to purchase ten apples for $10 and the apple merchant gives Shimon 11 apples, the additional apple is split between Reuven and Shimon (C.M. 183:6).

One explanation for this is that it is unclear whether the merchant intended to give the additional apple to the one paying for the apples (Reuven) or to the agent (Shimon). If the merchant said that he specifically wants the agent to receive the additional apple, it would belong exclusively to the agent (Rema ibid.). Alternatively, other Rishonim explain that since Shimon would not receive anything without Reuven’s money, even if the merchant wants Shimon to have the apple, it must be shared with Reuven (Shach and Taz ibid.).

There is also a disagreement regarding the halacha when the merchant gives a discount rather than an extra apple, charging only $9 for ten apples. One suggestion is that granting a discount is not comparable to giving something extra, since the agent never received something that he could claim was his. Any money that the agent does not spend belongs to the principal (Pischei Choshen Pikadon 11:[29] citing Maharya HaLevi 2:123).

Another perspective is that the merchant sold nine apples for $9; the tenth apple is the extra given by the merchant that the principal and agent would share (Minchas Shai 18, cited by Divrei Geonim, 98:11 and Minchas Tzvi, Poalim 8:2).

At first glance, it seems that your question is subject to the above dispute. You acted as the agent for the donor to purchase a Shas and the bookstore gives you, specifically, a discount. Accordingly, one opinion requires you to pass the entire discount to the donor; the second approach permits you to keep half of the discount for the yeshiva.

This approach, however, is not correct, since in your case you are not purchasing any sefarim for the donor that would make you be considered his agent. The agreement is that the donor intends to provide a Shas for the yeshiva, whatever the cost. That said, you are not allowed to keep any money for yourself, even though the bookstore gives the discount explicitly for you. However, if the donor pledged to pay the cost of a Shas at the going rate, you can go to the store and purchase a Shas for yourself at the discounted rate that they give you and then sell that Shas to the yeshiva at the standard price.

Furthermore, this applies only when the donor wanted specifically to purchase a Shas, but if his intent was to buy the zechus (merit) of purchasing a Shas for the yeshiva, it is understood that there is a standard rate that is charged for that zechus and whatever funds are not necessary to purchase a Shas are deposited to the sefarim fund.