Is taking the financial difference between the cost of hotel parking and that of a more distant, less expensive parking facility, while charging the higher cost to the company, considered unlawful (haram) or lawful (halal)?

You are not permitted to take payment for parking your car unless you have actually paid it, because that is deception and consuming wealth unlawfully. Allah Almighty says: (O you who have believed, do not consume one another's wealth unjustly), and the Prophet, peace and blessings be upon him, said: (Whoever deceives us is not of us). You must inform your employer of the truth. However, if your employer gives you more than you need as a donation, there is no sin upon you in taking it.