How can one atone for the sin of secretly taking money from an employer—due to insufficient salary—especially after many years have passed and the employment has ended?

The low wages of workers and the harshness of the employer do not justify the worker taking the employer's money, because it is an act of aggression and unjustly devouring people's wealth; as Allah Almighty says: "O you who have believed, fulfill [all] contracts." The questioner must return what he took to its owner by any possible means without informing him. Returning it is sufficient to absolve him and complete his repentance, and he must do so before performing 'Umrah.