Is it permissible for the husband to reclaim his apartment, which he gifted to his wife, knowing that he swore an oath not to enter it after a dispute, and is the wife selling the apartment and him buying it for her considered a circumvention of the oath?

The majority of scholars hold that the intention of the swearer, if he is not swearing an oath that is inherently general and encompassing, specializes the general and restricts the absolute, provided the wording allows for it. So, if the swearer intended in his oath not to enter a specific apartment, he may enter another apartment. His wife can sell this apartment and buy another, or lend him money. However, if his intention was not to enter any apartment belonging to her, then selling this apartment and buying another will not be beneficial, and he will break his oath by entering any apartment as long as it is owned by her. If he had no intention, the matter reverts to the reason for the oath and what provoked it. If the reason related to the house, the oath is confined to it. If it related to the woman, the oath includes cohabiting with her in any house. As for the phrase "Upon me be divorce," it is an oath by divorce. And the hadith of Umar regarding the purchase of charity is a prohibition against repossessing charity, which is mildly disliked (makrooh tanzeehan), and it does not invalidate the purchase.