Is it permissible to purchase the device at its well-known market price after paying a down payment to someone who claimed not to profit from it, but then it became apparent otherwise, and am I sinful for that, and how do I absolve myself?

The questioner appointed the man to purchase a device, so he is an agent and must offer sincere advice to his principal. It is not permissible for him to buy at a price higher than the customary market price. If he does so knowingly and willingly, he is a betrayer and a deceiver. He must repent and return the excess amount beyond the customary market price. If the agent purchases at a price higher than the customary market price, which is not usually overlooked in transactions, then the questioner is only obligated to pay the customary market price.