Is it permissible to give money instead of fasting for two days, and how much would that be, for someone who vowed to fast for three days and fasted for one day, intending to fast the remaining two days after childbirth?

If no time is specified for fulfilling a vow and an impediment arises, then it is permissible to delay it until the impediment is removed. You should delay fasting the two vowed days until you give birth, and you are not required to pay any money for them. If the inability [to fast] is permanent, then you must offer an expiation for breaking an oath (kaffarat yamin). If you are unable to [fast] due to a temporary impediment that is expected to be removed, you should wait for its removal. If the inability continues and becomes unlikely to be removed, then you must offer the expiation. If you pass away before fulfilling the vow, it is recommended for one of your guardians to fast the two days on your behalf, by analogy with settling a debt, and based on the hadiths that permit a guardian to fast on behalf of the deceased.