What is the ruling on one who vowed to give a monthly sum in charity on behalf of her mother but is unable to fulfill it?

Whoever vows obedience to Allah, it is obligatory for him to fulfill it, for the Prophet (peace and blessings be upon him) said: "Whoever vows to obey Allah, let him obey Him." If he is unable to fulfill his entire vow but is able to fulfill part of it, then he is bound by what he is able to do, and what he is unable to do is waived, in accordance with the rule: "What is achievable does not fall due to what is difficult," and according to the hadith: "When I command you to do something, do as much of it as you can." However, making vows is disliked.