Is it permissible to use the excess balance from the mobile phone allowance provided by the company, which was approved to improve the salary at the employee's request, and give it to the wife?

The matter requires detail. If your employer entrusts you with the balance and requests that it not be spent on anything other than necessities, then it is not permissible to give the excess to your wife or others without their permission. However, if the company pays you the balance monthly for use in work and other matters, and the excess is yours, then there is no harm in you benefiting from it as you wish, such as giving it to your wife. The determining factor is what was agreed upon, based on the Almighty's saying: "O you who have believed, fulfill [all] contracts," and the Prophet's (peace be upon him) saying: "Muslims are bound by their conditions." In case of doubt, one must refer to the person in charge at the workplace.