Is fasting three days as an expiation for an oath sufficient, knowing that after fasting, what could be given as expiation by feeding became available?

Atonement by fasting is not sufficient except for those who are unable to atone by feeding, clothing, or freeing a slave. Whoever does not find any of these three options may then resort to fasting for three days. This is based on the Almighty's saying: "Its expiation is the feeding of ten poor people from the average of that which you feed your [own] families or clothing them or the freeing of a slave. But whoever does not find [or afford] – then a fast of three days. That is the expiation for oaths when you have sworn."

If you were able to feed but were forgetful, the fast you undertook is not sufficient; rather, you must provide the feeding. It is stated in Al-Jawharah Al-Nayyirah: "If he fasted for his oath's expiation, and in his possession was his slave whom he had forgotten, or food that he had forgotten, then he remembered afterward, the fast is not sufficient by consensus; because Allah the Almighty conditioned that upon non-existence, and this person is existing [in terms of ability]."

If your father stipulated that you spend the money for a specific purpose, then it is not permissible for you to give charity from it or to use it for expiation, as this would violate the condition of the donor.