What is incumbent upon me when breaking an oath, if I swore "by al-haram," intending an oath and not a prohibition or divorce, that my wife would no longer be under my marital bond if I did such and such?

If you swore by the forbidden, intending it as an oath, then your breaking of the oath depends on two matters: your wife doing what you forbade her from, and your not fulfilling what you intended by your statement: "You will not remain married to me." If by this phrasing you intended the divorce to occur upon her doing the action it was conditioned upon, then the divorce has occurred, and you have not broken an oath. But if you intended it as a promise of divorce, then either you divorce her, and no expiation is binding upon you, or you pay the expiation for an oath and do not divorce her.