Is it obligatory to give away as charity the surplus money from a vow to feed a fasting person for 30 days, after covering the cost of the original vow, or is it permissible to take it?

A vow to provide iftar for fasting individuals is an act of obedience that must be fulfilled. The reference point for a vow is the intention of the one making it. If you intended to provide iftar for a fasting person for 30 days using the salary of two months, then you have fulfilled your vow. However, if you intended to spend both salaries entirely on providing iftar for fasting individuals, then you must spend the remainder of the second month's salary.

If the money was not spent by the end of the month, and if the intention was to provide iftar during Ramadan, then the time has passed, and you are obligated to provide iftar for fasting individuals with the money along with an expiation for breaking an oath (kaffarat yamin). However, if the intention was to provide iftar for any fasting individuals, then the money can be given to provide iftar for them after Eid without the need for expiation.