How can one continue preparing daily meals for one's father if he insists on paying for them, despite the questioner having sworn an oath of divorce not to take payment for them?

Whoever says: "Divorced be my wife if I do not do such-and-such," has made his wife's divorce contingent upon doing that thing. If he does it, the divorce occurs. This is the view of the majority of scholars. If he breaks his oath, divorce occurs, whether he intended it as an oath or to effect divorce. As for Sheikh al-Islam Ibn Taymiyyah, he holds that a contingent divorce that has the meaning of an oath does not occur if he did not intend to effect divorce, and an expiation for an oath becomes due. Accordingly, if the father insists on paying, one of your brothers can take the money and then return it to him or spend it on him, so that you may be safe from breaking the oath, and because the son's money is from his father's money.