Is it permissible to give a financial vow (nathr) to a relative who was previously promised financial assistance for marriage, such that this vow is counted as part of the promised amount?

Whoever vows obedience to Allah must fulfill it, for Allah the Almighty says: "And let them fulfill their vows," and the Prophet, peace and blessings be upon him, said: "Whoever vows to obey Allah, let him obey Him..." If the vow is absolute and not restricted to a specific cause, it must be given to the poor and needy. It is permissible to give it to your poor relative whose maintenance is not obligatory upon you. Fulfilling a promise is among noble character traits, but it is not obligatory to fulfill it according to the majority of scholars.