Is it permissible for a salesperson in a store to sell goods at relatively high prices and keep the profit for themselves, knowing that the owner of the store is unaware of this, and only one of the partners knows?

The uncle's argument for selling his son's goods for more than their value does not permit him to take the excess for himself; the surplus earned by the agent reverts to his principal, because he is entrusted with what is under his hand, and he must act in the principal's best interest. Taking this surplus without the partners' permission is a betrayal and an unlawful appropriation of people's money. Consequently, what the cousin sold at a price exceeding what was stipulated for it belongs to the shop and to all the partners. If you were to do the same, you would not own the surplus yourself; rather, it belongs to the shop, and you are not permitted to take it. The cousin must return what he took from the company's money unless your father and uncle absolve him of it.