Is it permissible for a purchasing manager in a hospital to buy medicine at a lower price from one vendor, then sell it to the hospital at a higher price without the administration's knowledge, while issuing an invoice from the original vendor, knowing that the purchasing manager will receive a commission from the vendor?

You are not permitted to take a commission from the drug seller except with the permission of the hospital administration, because the commission was offered to you due to your work. Similarly, you are not permitted to purchase medicine for yourself and then sell it to the hospital except with the administration's permission, because the administration prohibits that. You must fear Allah and not seek provision through deceit and fraud.