Have I sinned by purchasing oud oil at a low price from a worker when the shop owner was not present, and I later learned that its true price is much higher, and I did not return the oil?

The employee is an agent for the shop owner, and an agent acts in a way that serves the best interest and benefit. If the agent sells with an excessive deception, the principal has the option [to revoke the sale]. If the sold item remains, it is reclaimed. If it is lost, the principal is compensated for its value. The employee must remember the hadith of An-Nawwas bin Sam'an: "Righteousness is good character, and sin is what troubles your conscience and you dislike for people to know about it," and the hadith of Wabisa: "O Wabisa, consult your heart. Righteousness is what reassures the soul and comforts the heart, and sin is what troubles the heart and wavers in the chest, even if people give you a fatwa (religious legal opinion) and they issue you fatwas." And that a servant does not reach the reality of piety until he abandons what troubles his conscience.