Is it permissible to take the 3,0 pounds in this manner after reducing the agreed-upon amount from 15,0 to 12,0, and then informing the company of its reduction to 15,0, by agreement with the other party?

You have not clarified to us the basis of the employees' claim for the amount exceeding what was agreed upon, for us to determine their entitlement to it. The man is an agent for his employer and only has authority to act within what his principal has permitted him. If the manager authorized him to take what was less than 15,0, there is no sin upon him. However, his resorting to lying raises an issue. An agent is not permitted to take the difference for himself without his principal's permission. The employee must inform his employer of the truth. If the employer permits him to take the increase, he may take it; otherwise, he has no right to it.