Is it permissible to purchase a new server for the company with my own money, taking advantage of a special offer for new customers, and then sell it to the company at the same monthly price it was previously paying, knowing that there is a difference in cost due to the discount?

Your purchase of the server with your own money, and then selling it to the company for a profit, if the company agrees to it, is permissible. This is based on the Almighty's saying: "O you who have believed, do not consume one another's wealth unjustly but only [in lawful] trade [resulting] from mutual consent among you." (Quran 4:29), and His saying: "and Allah has permitted trade." (Quran 2:275).