What is the ruling on slave ownership in light of the questioner's desire to marry his Chinese colleague, who refuses to marry him due to her knowledge of his other wife?

You must cut off your forbidden relationship with that woman. You either choose the ruling of Allah and His Messenger by marrying her, if she is Muslim or from the People of the Book, or you leave her if she does not accept what is lawful or if she is otherwise. The concept of "milk al-yamin" (right of possession) has no relation to what you mentioned; rather, it is a forbidden and sinful relationship. So, fear Allah and do not disobey Him.