Is it permissible to pay off a debt on behalf of the deceased by giving charity on their behalf, instead of giving it to the heirs?

You must return the deceased's debt to their heirs. It is not permissible to give this money as charity on behalf of the deceased or their heirs as long as you are able to return it. If you are embarrassed to inform them that you were indebted to their deceased relative, you can return the money to them in a way that does not reveal you as the debtor, such as telling them that this money was a debt owed to their father by a man who appointed you to deliver it to them.