Is it permissible to take money for selling a computer indirectly, while deliberately labeling the amount as "maintenance cost" instead of "purchase price"?

It is not permissible to take money in exchange for repairing and maintaining a computer that was relinquished to the manager, as this constitutes deception, falsehood, and consuming wealth unlawfully, especially since you declared that you relinquished it to him for free. You must adhere to the conditions, for Allah Almighty says: "O you who have believed, do not consume one another’s wealth unjustly but only [in lawful] business by mutual consent." And the Prophet, peace and blessings be upon him, said: "He who deceives is not of us," and "Muslims are bound by their conditions."