Is a wife considered sinful, Islamically, if she does not return the apartment that served as a dower, after obtaining a divorce by judicial decree?

If the apartment was part of your wife's dowry, and the khul' (divorce at the instance of the wife) stipulated that the woman relinquish the dowry, then she must return the apartment to you, based on the hadith (definition) narrated by Al-Bukhari concerning Thabit ibn Qays and his wife.

The judge's ruling, based on the submitted documents, does not permit the woman to conceal any part of the dowry that was not formally documented. The Prophet, peace be upon him, warned against taking another's right based on a judge's ruling that was made based on outward appearances, as he said: "Indeed, I am a human being, and you bring your disputes to me. And perhaps some of you are more eloquent in presenting their case than others, so I judge in his favor based on what I hear. So, whoever I rule in his favor concerning a right belonging to his brother, let him not take it, for I am only cutting off for him a piece of fire."

A judge's ruling does not make the unlawful lawful, nor does it negate the right to claim. Therefore, the wife and her family must abide by what Allah Almighty has enjoined and avoid consuming wealth unlawfully.