What is the ruling on not fulfilling my mother's vow to marry off her daughter for 7 days for 7000 Omani Riyals, in order to facilitate her marriage in light of the high dowries?

Facilitating marriage by not exaggerating dowries and wedding expenses is commendable and blessed for the couple. It helps young people maintain chastity, and doing otherwise is extravagance and forbidden by Islamic law. As for vows concerning permissible acts, one is not obliged to fulfill them. Rather, the one who made the vow has the choice to either perform the act or abandon it, with the obligation of offering an expiation for an oath.