How can one be absolved of a sum of money that a mother deposited as a trust with her daughter, who then lent it to her son, and he did not return it, and the mother passed away more than twenty years ago?

This money is from the mother's inheritance. If the son returns it to his mother, she should pay the excess beyond her share to the heirs, each according to their share. If the son does not pay the amount, then she should demand it from him, or at least the excess beyond her share.

If she lent the money with her mother's permission, she is not liable, and the son must return it to the heirs. If she lent it without her mother's permission, she is liable if the son does not pay the money, due to her transgression, and she must seek forgiveness and repent.