Is it permissible to give as charity the amount withdrawn from the mother's money before her death, or must it be disposed of in another way to absolve oneself, given the presence of other heirs?

One must repent to Allah Almighty for taking the mother's money without her permission. Part of perfect repentance is to return the money to her heirs. It is not permissible to give it as charity on her behalf. Al-Ghazali stated that money with a specific owner must be given to him or to his heir. If the mother has no heirs other than her children, the money is to be divided among the siblings, with the male receiving the share of two females.