What is the ruling on taking a small sum of money from a company without the knowledge of its owners, if this is due to injustice and oppression, and is that money considered unlawful, necessitating its return even if its return is impossible?

The relationship between the questioner and the company is a special ijarah (lease/hire) relationship. Ijarah is a binding contract that neither party can unilaterally dissolve except with the consent of the other party or upon the expiration of the term. If the company manager dismisses the questioner before the contract term ends without cause, the company is obligated to pay the questioner's salary until the contract's expiration date. The questioner has the right to demand from the company the salary for the remaining period of the contract. As for the amount in her possession, if she has a right to it from the company (as part of her entitlements after arbitrary dismissal), then she may keep it. Otherwise, she is obligated to return the amount to the company by any means that ensures its delivery to them, along with repentance to Allah.