What is the ruling on one who buys a mobile phone top-up card and a monetary value greater than the value of the card purchased is added to his balance?

If the phone company does not announce a free credit increase, then the increase that occurs is considered an error, and whoever discovers it must inform the company to correct it. It is not permissible for him to benefit from it except with the company's permission and consent, based on the Prophet's (peace and blessings be upon him) saying: "The wealth of a Muslim person is not lawful [to take] except with his willing consent." However, if the increase is with the company's desire and consent, such as when it announces an additional credit top-up upon purchasing a card, then there is no harm in that, and it falls under the category of price reduction.