Praise be to Allah.
The commission that a worker takes for selling and so on must be agreed to, because it is regarded as part of the salary, and because not agreeing on it usually leads to conflict.
The company is not obliged to give its employees the usual, customary commission. Rather that should be done according to agreement, and the company may give similar commission, or less or more than that.
You have to speak to the manager of your company and come to an agreement with him about commission, then you can ask for it if you do the work that entitles you to it.
It is not permissible for the one who has been appointed to sell something to increase the price except with the permission of the one who appointed him, because the one who has been appointed is restricted in his actions to that for which he has been given permission.
It is not permissible for him to increase the price so that he can take the difference. This comes under the heading of cheating and consuming wealth unlawfully. Rather any profit made must be given to the one who appointed him, and the one who has been appointed is only entitled to the wage or salary that was agreed upon.
The scholars of the Permanent Committee for Ifta’ were asked: There is a man who sells goods for another man; in other words, he gives him a product to sell it for him with his knowledge. This man increases the price, and takes the extra amount for himself. Is this regarded as riba, and what is the ruling on the one who does that?
They replied: The one who is selling the goods is regarded as an agent of the owner of the goods, and he is entrusted with the goods and with the price he receives for them. If he takes something of that price without the knowledge of the owner of the goods, then he is betraying the trust, and what he has taken is haraam for him. End quote from Fataawa al-Lajnah ad-Daa’imah (14/274).
If there has been a clear agreement on a specific rate of commission, then the company refuses to give it to the employee, and he cannot find any legitimate means of taking it, but then finds and opportunity to take something from the company’s wealth, it is permissible for him to take as much as he is certain that he is entitled to of this wealth, according to what is known to the scholars as the issue of zafar (opportunity to take one’s due).
But it is not permissible to increase the price of the product, because this is contrary to the terms of agency [being appointed to sell the product on behalf of the owner], and it is obviously a transgression. Rather the discussion about using an opportunity to take something from the company’s wealth – such as cash that one receives from customers and the like – is limited to that which will not expose one to accusations and punishment, which is one of the conditions with regard to the issue of zafar (opportunity to take one’s due).
And Allah knows best.