Is it permissible to take and benefit from money that was returned by someone who had received it as assistance, but at the time it was not specified whether it was zakat al-mal or sadaqah, knowing that the giver is currently in need of the money?

If the money is Zakat, it is not permissible to own or benefit from it; rather, it must be given to those who are entitled to it. If it is charity (sadaqah), the understanding from the Malikis' statements is that it is permissible to benefit from it if it was designated for a specific person who did not accept it, by analogy with the ruling on a piece of bread offered to a beggar. However, since the status of the money is doubtful, being between Zakat and sadaqah, it is more prudent and cautious to give it to the poor and needy.