Is it considered sinful for a partner to conceal from his senior partner that he is undertaking projects and tenders independently, knowing that he gained access to these projects through one of the company's clients, and is the other partner entitled to demand a share of the profits?

The company in question is a partnership (shirkat al-inan), which is permissible by consensus. It is permissible for a partner to receive a fixed salary for specific work done for the company, through a separate contract from the partnership agreement. It is also permissible for a partner to use his private funds to enter a tender related to another company that has no connection to the existing business between him and his brother, provided that no use is made of the company's tools or rights, that it does not distract him from his work, and that it does not result in harm to the company. He is not obligated to inform his brother of this. However, if the tender is specific to the company owned by the two partners, it is not permissible to enter it without the knowledge of the other partner, whether directly or fraudulently, because a partnership (shirkat al-inan) is based on agency (wakalah) and trust (amanah).