Is fasting considered an expiation for a vow or a voluntary act if a person vowed to fast for a month and achieved what he desired, and what is the ruling on this vow?

Whoever vows obedience to Allah by fasting, conditioned upon the occurrence of something, must fulfill the vow if the condition is met, based on the Prophet's (peace be upon him) saying: "Whoever vows to obey Allah, let him obey Him." In this case, his fasting is obligatory due to the vow, not voluntary.