What is the ruling regarding someone who vowed to donate a red chicken but could not find one, and does he fulfill his vow if he substitutes it with another chicken of the same price, and how does he fulfill his vow if he cannot find the red one?

Fulfilling a vow (nadhr) is obligatory if it is an act of obedience to Allah, but not if it is otherwise, for the Prophet, peace and blessings be upon him, said: "There is no vow except in that by which the face of Allah is sought." If one vows something restricted by time, place, person, or type, and then gives it away in charity in a manner other than what was specified, then their vow becomes void. Therefore, the man must give a chicken in charity, regardless of its color, as color is not considered an act of proximity (qurbah to Allah).