What must an employee do if he has been negligent in his work for more than ten years and received a salary for tasks he did not complete, and he is now regretful for that, knowing that he is poor and in a bad financial situation?

You must seek forgiveness from your employer, regretting what has passed. If they permit you, then there is no blame. Otherwise, you must return what you took for work you did not perform. If you are unable to return it directly to your employer, you may spend it on the poor and needy.