Ruling on the agent taking an increase in exchange for the work entrusted to him

Question:
I work in a workshop, and a representative from a factory came to repair a crane. When we performed the maintenance, the technician who did the maintenance set the price. Then he came to fix the defect, and another worker came and said: "This one doesn't understand; raise the price?"

Answer:
Another worker came?
Q: Yes, and with him was the factory's representative.
The Shaykh: Of the same kind as the first incident?
Q: Yes.
The Shaykh: Meaning, he said, "Increase it."
Q: Yes, he said, "Increase it," and the factory's representative was not... increasing it?
The Shaykh: The agent?
Q: Yes.
The Shaykh: Meaning, he plays around, once like this, and once like that.
Q: So when I added one thousand riyals and took the original invoice, they said, "Return the invoice price to eight hundred riyals." When I asked him about that, he said: "We have an agreement with the factory's representative to give him 10%, and we extract it."
The Shaykh: Does the agent take the increase?
Q: Yes.
The Shaykh: This is not permissible. There should be one price. If the work is the same: one price. It is obligatory for the agent to fear Allah and fulfill the trust. His work should be the same; he should not play around, taking from this one and taking from that one. It is obligatory for him to fear Allah and for the wage to be the same for what the factory owner has agreed upon. And if the factory owner gives him his wage—whether monthly, daily, or annual—rather than him playing around with people: this one says eight hundred, and this one says one thousand. This is not permissible. This is playing around.