If he markets a product that he does not possess, does the profit belong to him?
Question

A person who works in some company has agreed with my brother to market a product he has. Then to divide its price between them. Knowing that he got this product due to a mistake he did in the accounts that he sends to the company. My question is: I am managing this business for my brother. Is the money we earned from this business Halal? What is the ruling on the profit we made from selling this product? Is the money I earned through managing this business for my brother Halal?.
Praise be to Allah.Firstly: 
If your friend obtained the product as the result of a mistake that he made in his accounts, as you say, then this product is not permissible for him, rather it belongs to the company and it is not permissible for anyone to help him in selling it. What he has to do is correct the mistake and return the product to the company. 
Secondly: 
Whatever is taken unlawfully is regarded as coming under the same ruling as property seized by force, and must be returned to its owner if it is still in existence, otherwise something similar should be given to him if something similar exists, otherwise its value should be given to him. 
If the one who stole it deals in it and makes a profit, then he must return this profit to the owner of the item, because it is the profit on his wealth. 
Shaykh ‘Abd al-‘Azeez ibn Baz (may Allah have mercy on him) was asked about a man who had some money left in trust with him, and he invested it and made a profit, then he returned the money to the owner without the profit. 
He replied: 
If someone entrusts you with something, you have no right to dispose of it without his permission, and you must look after it as similar things are looked after. If you dispose of it without his permission then you must seek his forgiveness. If he forgives you, all well and good, otherwise you must give him the profit of his wealth or come to some agreement with him to share it equally or whatever. Agreements between Muslims are permissible except an agreement that makes a halaal thing forbidden or a haraam thing permissible. End quote from Fataawa al-Shaykh Ibn Baz (19/411). 
Thirdly: 
The wages that you received for facilitating this trade must be given in charity, because you received them as the result of a haraam action. 
We ask Allah to enable you to repent sincerely. 
And Allah knows best.