It is permissible to work as a customs broker in return for payment, subject to the condition that the goods handled are things in which it is permissible to trade.
With regard to what you say about this amount of cement, if it is something that is allowed, then there is nothing wrong with you marketing it in return for payment.
This work of yours comes under the heading of brokerage (samsarah), i.e., acting as an intermediary between the seller and the buyer. In the answer to the question ( 45726 ) we have explained that working as a broker is permissible, and we have discussed the scholars’ opinions on this matter.
If you take the cement from its owner and sell it to the buyer yourself, then you are an agent of the seller, and there is nothing wrong with an agent accepting payment in return for his work.
Ibn Qudaamah (may Allaah have mercy on him) said in al-Mughni (7/204):
It is permissible to appoint an agent or delegate someone to do something with or without payment, because the Prophet (peace and blessings of Allaah be upon him) delegated Unays to carry out hadd punishments and ‘Urwah to buy a sheep without giving them any payment, and he used to send his agents to collect the zakaah, and he paid them for that. Hence two of his cousins said to him: “Why don’t you send us to collect this charity and give us what you give the people, and we will collect for you what the people (i.e., the agents) collect, and we will earn wages like they do.” Narrated by Muslim, no. 1072.
So if a person is appointed as an agent to buy or sell, then he is entitled to payment for his work. End quote.
There is nothing wrong with making the wages of the broker or agent a known percentage.
Shaykh Ibn Baaz (may Allaah have mercy on him) said:
There is nothing wrong with paying a fee to the broker for finding a seller or a buyer, and there is nothing wrong with stipulating this payment. End quote.
Fataawa Ibn Baaz, 19/31
The Standing Committee was asked:
There is a great deal of discussion about the fees charged for finding buyers and sellers. Some charge 2.5% and some charge 5%. What is the price prescribed in sharee’ah? Or is it something to be agreed upon by the seller and the broker?
If the broker, the seller and buyer agree that the broker will be given a certain fee by the buyer or the seller or by both of them, that is permissible. There is no set limit for this fee, rather whatever is agreed upon by the one who pays is permissible. But it should be within the limits of what is customary among the people, it should benefit the broker in return for his work as an intermediary between the seller and the buyer, and it should not harm either the seller or the buyer by being more than the customary price. End quote.
Fataawa al-Lajnah al-Daa’imah, 13/130
They also said:
It is permissible for the broker to take a fee which is a fixed percentage of the price of the goods in return for his work, from the seller or the buyer, according to whatever has been agreed, without any injustice or wrongdoing. End quote.
If it is a percentage of the profit and not of the price of the goods, then the Hanafi scholars have stated that this is permissible, and that it is akin to mudaarabah (profit sharing). Mudaarabah is when a man gives his money to someone who will trade with it in return for a percentage of the profits. See Mataalib Ooli al-Nuha, 3/543; Kashshaaf al-Qinaa’, 3/615
The point is that there is nothing wrong with you accepting the agreed-upon percentage (commission).
And Allaah knows best.