Is it permissible for an engineer to purchase a service for his private company at a lower price than the supplier, then sell it to the company he works for at a price lower than its current price, thereby making a profit for his private company and benefiting the company he works for, knowing that he can negotiate directly with the supplier to reduce the price for the parent company without making personal gains?

If the questioner is not a sales or procurement employee, and not an agent for their employer in purchasing the service, then there is no objection for them to buy the service for their private company from the provider, and then sell it to their employer and benefit from the price difference. What is prohibited is for the questioner to combine both parties of the contract, becoming both the seller and the buyer's agent at the same time.