Is it permissible to count the amount the questioner lent to his neighbor as part of the vow designated for the poor? And will fasting three days a month suffice him for the lapsed period? And is an expiation for an oath incumbent upon him for delaying the fulfillment of the vow?

Fulfilling a vow of obedience is obligatory. Therefore, donating the vowed amount and fasting the vowed days are required. It is not permissible to waive a debt owed to you by a man and consider it as fulfilling the obligatory charity, because charity necessitates transfer of ownership, whereas this is a waiver. The vowed days that were not fasted must be made up, and there is no expiation due upon you for delaying their fulfillment.