Is it permissible to buy an apartment located above a Christian's residence, given that the apartments are independent and there is no direct interaction?

There is no harm in purchasing the aforementioned apartment, even if a Christian lives below it or next to it. Muslims coexist and intermingle with the People of the Book. This is more likely to win their hearts and endear Islam to them. A neighbor has rights, whether Muslim or non-Muslim. The Prophet, peace and blessings be upon him, emphasized the importance of neighbors. Abdullah ibn Amr had a Jewish neighbor, and he used to give him gifts from his slaughtered animals. The residents of the building are encouraged to invite this Christian to Islam and treat him well, so that Allah may guide him.