Is it permissible to withdraw from a monthly donation commitment that has not yet been paid, knowing that it is neither a vow nor a seized gift?

If the commitment to donate is not accompanied by the legal formula of a vow (nadhr), then it is not considered a vow. A vow must have a formula that indicates the mukallaf's (accountable person's) commitment to an act of proximity to Allah, such as "It is Allah's due upon me to do such and such," or "I have a vow to do such and such." If what was issued was merely a promise of righteousness or a gift, then the person who made it may retract it, although doing good is more preferable.