What should I do to redress injustices and repent after the death of the owner of money that I took, knowing that she has no legal heir and I cannot determine the amount? And do you advise giving charity on her behalf?

You must accurately determine the amount of money you took. Then, if any of it was from this woman's money, acquired through permissible means, or if the source of its acquisition is unknown, and the woman died without a legal heir, you must give it away in charity on her behalf. If the money was a result of usurpation or theft, it must be returned to its owners or their heirs. If that is impossible or the owners are unknown, then give it away in charity on their behalf. However, if it was from unlawful dealings such as usury or selling alcohol, then it must be disposed of by spending it on the welfare of Muslims, such as assisting the poor.