Is a wife forbidden to her husband if she spends from her own money on the house after he told her, "You are forbidden to me if you spend from your money on the house," and is expiation (kaffarah) obligatory upon him for that?

If you made your wife unlawful to you based on her spending from her own money on the house, then if she does not spend, there is nothing upon you. But if she does spend, you have broken your oath. Your intention is to be considered: if you intended divorce, it is a divorce; if you intended ẓihār, it is ẓihār; if you intended īlāʾ, it is īlāʾ. If you intended it as an oath, then an expiation for an oath is binding upon you. And if you did not intend anything, then an expiation for an oath is also binding upon you.