What is the ruling on taking a sum of money in exchange for securing hospital supplies, knowing that their price is less than the market price, and is this considered theft or embezzlement?

You are not permitted to take the price difference when purchasing parts as an agent for the hospital, because any increase is for the hospital and any decrease is against it, unless you have been authorized to possess what exceeds the purchase price. Otherwise, taking it is considered deceit and treachery, as Allah Almighty says: "And do not transgress. Indeed, Allah does not like transgressors" (Al-Baqarah: 190). An example of this is what Urwah ibn al-Ja'd did when he bought two sheep for a dinar and sold one of them for a dinar. The Prophet, peace and blessings be upon him, approved of that. If the increase had been his right, the Prophet, peace and blessings be upon him, would have taken it.