Is the amount earned from issuing a transportation invoice at a price of 35 instead of 31, based on an agreement with a friend who works as an accountant in another transportation company, knowing that my friend paid in cash at the lower price, considered permissible (halal) or forbidden (haram)?

It is not permissible for an employee to dispose of anything without authorization. He must inform his employer or the requesting party that transportation is not available. It is not permissible for him to act unilaterally and add that to his company, because it is an تجاوز (transgression). What is taken to circumvent invoices is considered bribery, and its doer is cursed. The employee must return all the money to his employer unless given permission by them. And whoever took the money from him must return it to him so that he may hand it over to his employer, or he may hand it over directly himself.