[Below are two similar questions we received orally within 12 hours of each other from people who are very careful about other people’s money, as we encourage.]
Question 1: My yeshiva entrusted me (a kollel student) to arrange an oneg Shabbat for the talmidim. I was to responsibly buy refreshments and be reimbursed based on receipts. There is a significant amount of leftover food, some of it in open packages and some untouched. Can I or other participants use that food, or should I give it to the yeshiva. If keep it, may I ask full reimbursement of the purchases?

Answer 1: There are a few models to the possible nature of your arrangement with the yeshiva, which would impact elements like the ones you ask about.
You could have been serving as an agent (shaliach), buying food on the yeshiva’s behalf. If so, they have to reimburse you in full for what you bought as their agent, and the food is theirs. Then you would have to determine whether they allow you to eat their food after the time during which they clearly gave permission (during the oneg). One may assume they would be happy that you finish small amounts from open packages. Regarding the rest, it likely depends on various factors, including the management style of the yeshiva and the extent to which it is worthwhile for them to store the food until the next event. Even in cases where one is confident the owner of an object would be happy with a friend taking his object, there is an unresolved machloket whether it is permitted (Shach 359:5) or forbidden (Tosafot, Bava Metzia 22a) to do so (see Living the Halachic Process vol. II, J-2, where we preferred refraining from use).
Another possibility is that you bought the food for yourself with a promise of compensation. If that is the case, the food is yours, and you can do whatever you want with it. However, it raises a different question: how much compensation can you ask from the yeshiva? If you do not take the food for yourself, then they probably have to compensate you for all you bought and cannot require you to use that which was not eaten at the oneg on your account. However, leftovers that you do want to use turn out to be things that you did not spend on the group, and it does not seem that you should ask for compensation for them. On the other hand, the value to you of the leftovers (certainly the open packages, but likely even some closed packages) may be less than the amount you paid in the store. Therefore, you would not have to reduce the full face value from your request of a refund.
We encourage stringency on matters of monetary ethics. The wisest stringency is often to raise the issue with the relevant authorities with a smile, hakarat hatov, and willingness to pay or forego, respectively. In cases of good relations and only a few shekels at stake, each side is usually generous. Asking permission not only removes a question of impropriety but likely gets the best deal in the present and builds trust for the future.

Question 2: I am a teacher who received 500 shekels to spend on a party for a group of my students. I am clearly expected to keep the leftovers. The generous budget enabled me to buy more expensive vegetables than I would not normally buy for myself. After further planning, I think a different salad will be more appropriate, which would make the expensive vegetables unnecessary. If I decide to not use them, I should "buy them" from the school, but they are not worth their cost to me. What should I do?

Answer 2: While the school might allow it, it is not so nice to ask the school to pay money for something that its students did not benefit from at all. On the other hand, you acted with good intentions, and there is no reason for you to lose money trying to do the nicest thing for your students and being honest. Sometimes "practical advice" augments halachic advice importantly. We suggest that you make the expensive salad even if you now think that you have a better idea. I am sure it will be fine, and it is worth it to avoid the moral dilemma.