What is the ruling on selling goods at one price, and requesting an invoice from the seller for a higher price to sell them at that price to the buyer without his knowledge? And does the ruling differ if the buyer is aware of that?

The broker's action, done without the buyer's knowledge, is forbidden because it constitutes consuming his money unjustly. It is also forbidden for the seller to assist him in this by falsifying the price in the invoice. The broker can stipulate a commission from the buyer for the purchase; if the buyer agrees and is aware of it, then there is no harm for the broker to take it. It is preferable to agree upon it before the purchase, and brokerage is permissible in Islamic law.