Is it permissible to dispose of half of the dowry that was returned after divorce, based on the premise that the purpose of the original gift, which was marriage, has been fulfilled?

A restricted gift is used for the purpose intended by the giver. Therefore, whoever is given money to marry with it is not allowed to use it for anything else. If the marriage takes place and then a divorce occurs, and a portion of the money remains, the remaining money must be returned to the givers or used for marriage, because the goal of marriage was not achieved.