What is the ruling on a man who said to his wife—while she was at her family's home—"I divorce you if you leave your family's home for a month, then you are divorced," intending by that a divorce, and also that he would not see her and she would not see him for the mentioned period? And how can he revoke that?

As long as you intended divorce with your oath, then if you break it, your divorce takes effect, and your wife becomes irrevocably divorced from you. Therefore, if you do not wish to separate from your wife, she must remain in her parents' home, not leave it, and you must neither see her nor be seen by her until the month passes. This way, you fulfill your oath, and the divorce does not take effect upon her.