What is the ruling on money acquired from an additional discount from European companies as a commission, while keeping the invoice at the original price without the partner's knowledge? Is this considered devouring people's wealth unjustly?

You are not permitted to take any money in excess of what was agreed upon with your friend who appointed you to purchase cars. This is because it constitutes consuming wealth unlawfully. You must return the amounts you took from him unless he absolves you of them. You are only entitled to the agreed-upon commission, because any additional profit earned by the agent reverts to his principal.