What are the consequences for someone who swore not to do a certain thing, then did it, knowing that he swore before doing this thing that he would not do it tomorrow?

If someone takes an absolute oath not to do something, and then, before doing it, wishes to restrict their abstention to "tomorrow" and swears to that effect, this restriction is not valid, because the exception is not connected to the oath. Consequently, the first oath is broken, regardless of the time of the action, and an expiation for the oath becomes due: feeding ten poor persons, or clothing them, or freeing a slave, or fasting three days for whoever cannot afford these.