Is what happened considered deception, necessitating an expiatory offering for a broken oath, and does the ruling on the oath change if the banknotes sworn upon are altered?

It appears that your intention in the oath was not to take the money, but rather the papers. Changing the papers does not nullify the expiation. Jurists have stipulated that a stratagem does not waive the expiation, such as someone who swears not to do something and then delegates another to do it; in such a case, he breaks his oath. You owe an expiation for breaking an oath: feeding ten poor people, or clothing them, or fasting for three days if you are unable to do the former. Obeying your mother and taking the money is better than refusing it because of the oath.