What is the ruling on the action of my colleague who takes a portion of the money paid by the client and records a lower value on the receipt, knowing that the administration has granted us the right to negotiate with the client provided it is not less than a specified value?

What your colleague is doing—forgery, breach of trust, and unlawfully consuming wealth—is impermissible. It is also impermissible to cooperate with him in that. You must advise him and remind him of the impermissibility of his actions, and threaten to report him to the administration. If he does not respond, then you are obligated to inform the employer.