If he buys a phone card and it has extra credit
Question

                What is the ruling on a person who buys a card for his mobile phone and after he refills it, more credit is added to it than the price that he paid for the card?.
Praise be to Allah.
    So long as the phone company has not announced the additional 
    amount and it is added to the card, then what has happened is the result of 
    some mistake, and the one who has discovered this has to inform the company 
    so that they can correct it. He has no right to benefit from the extra 
    amount unless he asks permission and gains the company’s approval, because 
    the wealth of others is not permissible unless it is given willingly; a 
    mistake does not make it permissible to dispose of this extra amount.

    The Prophet  (peace and blessings of Allaah be upon him) 
    said: “The wealth of a Muslim man is not permissible unless he gives it 
    willingly.” Narrated by Ahmad (20714); classed as saheeh by al-Albaani in 
    Saheeh al-Jaami’, no. 7662.  

    And he  (peace and blessings of Allaah be upon him) said: 
    “No one of you truly believes until he likes for his brother what he likes 
    for himself.” Narrated by al-Bukhaari (13), Muslim (45). 

    But if the extra amount is added to the card with the 
    approval of the company, such as if it announces that whoever buys a card 
    for one hundred, they will put one hundred and twenty on the card, for 
    example, then this comes under the heading of lowering prices, and the 
    seller and renter may do that. 

    And Allaah knows best.