Is the profit gained from selling goods to a merchant considered unlawful if he was informed of a price that includes profit instead of being informed of the real price, given his knowledge that a profit margin had already been taken from the original seller?

If the agreement between you is that your commission will be added to the price of the medical supplies, and you will not inform him of the original price, and he is aware of this; then there is no blame upon you. However, if he asks you to search for goods and inform him of their original price, and he will give you a separate commission; then it is forbidden for you to increase the price.