How can a person who fell short in his work make amends for his sin?
Question

                There is an employee who fell short in his work one month, but he took his salary in full, and he and his family made use of it, and he bought some academic and religious books to benefit from them. His conscience is troubling him about that shortcoming, and he began to think about how he can get rid of the books or give them away, and he intends to buy similar books from his salary in a month in which he does his work completely.

Should he get rid of the books?
Praise be to Allah.
    The employee is obliged to do his work as is required of him, so that he 
    will be entitled to the salary that he takes in return for that work. 
If 
    there is any shortcoming in his work, then the basic principle is that he is 
    not entitled to take the salary in full, because he did not do the work in 
    the manner required. 
It 
    says in Fataawa al-Lajnah ad-Daa’imah (vol. 1, 15/154): What is 
    required of the one to whom work has been given, in return for which he 
    receives a salary, is to do the work in the manner required. If he falls 
    short in that without a legitimate excuse, the salary that he takes is not 
    permissible for him, because he is taking it without having done his work 
    properly in return. 

    Based on that, you have to repent and not repeat what you have mentioned. 
    You must be honest in doing the work that has been given to you to do, and 
    give ing charity whatever is equivalent to what you took of the salary with 
    no legitimate excuse. End quote. 

    Based on the above, the first thing that anyone who does any such 
    shortcoming must do is repent to Allah, may He be exalted, from his 
    shortcoming. 
He 
    does not have to get rid of the books that he bought with this salary, or 
    give them away; rather he may make use of them. All that is required of him 
    is to get rid of the salary that he received, commensurate with the degree 
    to which he fell short. If he fell short in half of his work, he should get 
    rid of half of that salary, and so on. 
He 
    should only get rid of it by returning it to the workplace to which it 
    belongs, if that is possible; otherwise he may donate it to charitable 
    causes. 

    Note: some countries have set up accounts for employees of the state who 
    fell short or took anything from public funds, to pay it back. If someone is 
    in a country where such accounts and schemes have been set up, and he falls 
    short in his work and he is employed by the government, then he may return 
    the money to that account and thus will have discharged his duty. 
We 
    ask Allah to help us all to shoulder responsibilities and render back trusts 
    in the most perfect manner, for He is able to do that. 

    And Allah knows best.