What is the ruling on taking a sum of money from the company's client for myself, knowing that my manager used to deduct from my commission, which was agreed upon in the contract?

Taking money for yourself without handing it over to the company is not permissible; it is considered a breach of trust, and you must return this money to the company. What you mentioned about your colleagues having higher salaries does not justify taking the money, as it is not an injustice if you agreed to it in the contract. As for your claim that the manager reduces your commission, there might be a valid reason for that, and you must first prove your right before claiming it.