Is it permissible to purchase products at a special discount from suppliers due to the personal relationships of a company owned by the brother, and then sell them later to the parent company?

Your sale to your company has details:

1. If you are selling to the purchasing manager in your company and you disclose that you are a seller, there is no harm in that. 2. If you are responsible for purchasing for your company and you want to buy for yourself and then sell to your company without its knowledge, this is not permissible except with the company's permission. Otherwise, it is a betrayal of trust, because the origin of a sale is that it should be between two parties. The Malikis and Hanbalis permitted an agent to buy for himself with the principal's permission, while the Hanafis and Shafi'is prohibited it absolutely. 3. If you will buy at a discount and sell to your brother's company, and then your brother's company sells to your company, there is no harm in that, provided that you are not responsible for purchasing or pricing. The purchasing manager must act in the best interest of his company and seek the best prices.