Is the profit considered haram if my friend (who works at a cargo transportation company) and I buy spare parts for the company with cash and sell them to it on credit, knowing that my friend is a partner in the profit and that this is not my primary job?

It is not permissible to cooperate with the friend because he is a trusted agent, and his profit is a betrayal and an unlawful appropriation of money. He must return the price increase to the company, as stated by the author of al-Kafaf: "And if he increases [the price], the increase belongs to the principal, not to his agent who did not act justly." This is also consistent with what Urwah ibn al-Ja'd (may Allah be pleased with him) did with the Prophet (peace be upon him). Likewise, it is not permissible for him to take a commission for himself without the permission of his employer. He must fear Allah, and you should clarify this to him, advise him accordingly, and avoid dealing with him in matters of betrayal. Whoever gives up something for the sake of Allah, Allah will compensate him with something better: "And whoever fears Allah – He will make for him a way out and will provide for him from where he does not expect." If he refrains from taking the commission or if the company permits him, then there is no harm in selling the materials on credit after they have come into your possession.