I have been fired from my teaching job and the school owes me money. They admit to that. I have access to the place where the school stores some of its belongings, specifically Seforim. Can I take their Seforim and sell them to repay the debt? Can I take the ones that I could use and deduct it from the debt?
You may not take their belongings and sell them to repay the debt. This is considered stealing.
It seems to say explicitly in Choshen Mishpat that this can be done. Can you please provide sources for the claim that this is considered stealing?
Once the party agreed they are liable to pay and the payment due date was determined or the total sum owed was calculated, even if it was originally due to theft and even if they now don’t wish to pay any more, it’s considered a loan where there is a Lav to seize his belongings.
When dealing with an outstanding debt, any heter would only apply to the actual object that is yours . This too with some limitations. No one allows you to take his objects.
Even when permitted, it’s only with the intention to take the belongings to Beis Din to assess its value and collect from it. One may not take the belongings to sell them or keep them.
The Zohar, quoted by Poskim such as the Ketzos Hachoshen and others, uses very harsh words about one who holds back in returning a Pikadon because the owner owes him money. See also the references in קונטרס ענינה של תורת החסידות about the matter. This case is far more severe.
Even those who would permit taking his money, or even in some cases his objects, it is only when it was given to you as a Pikadon or a Mashkon, and only if the Pikadon preceded the debt, and not in the reverse. None of this applies here.
Furthermore, there is a specific Cherem Hakadmonim against seizing Seforim against a debt. Those who permit a Melamed to do so are referring to a case where he is seizing the Seforim he received in order to teach the children.
Poskim advise that even in cases where it is permissible, one should have the issue heard by an expert Rav who will hear both sides.