What is the ruling regarding the partner responsible for purchasing selling goods to his company at market price, while he buys them at a lower price based on his connections, without the knowledge of the other partners?

It is not permissible for an agent entrusted with purchasing goods for a company to buy them at a low price and then charge the company the market price, because this is a betrayal of his principals. The price difference belongs to the company, not to the agent, unless the partners explicitly authorize him to take it.