Is it permissible to write a lease contract for 210,0 Saudi Riyals, knowing that 30,0 Saudi Riyals of it is an agent's commission, which might be annual?

You are not permitted to do that, as it falls under aiding in sin and aggression, and consuming people's wealth unlawfully. The company's agent is not allowed to take this commission without the company's knowledge, and colluding with him to do so is prohibited.