Is it permissible to give charity to compensate for past accounting errors that led to a decrease in workers' salaries, if it is not possible to reach them to restore their rights?

If the error regarding the money was due to negligence, then one must repent and return the money to its rightful owners or their heirs. If unable to return it, then one should give it as charity on their behalf. If you later find them, give them the choice between accepting the charity or having their money returned to them, and the reward for the charity will be yours.