Is it permissible for a manager who has personal relationships with wholesalers to buy goods at a special discount, then sell them to the store at market price and benefit from the discount value without the knowledge of the heirs, while concealing his role as an intermediary?

An agent is not permitted to purchase for himself without the permission of his partners due to the suspicion involved, because his transaction combines two conflicting objectives: seeking a lower price for himself and securing the best price for the principal. What you are doing is a forbidden stratagem to deceive your partners and unjustly consume their wealth. The surplus that remains in your hand when purchasing below market price is not yours; rather, it belongs to the business and to all the partners. You must be truthful with your partners.