Is it permissible for a mediator to take a sum of money from the buyer in exchange for his mediation in purchasing cement?

What you have inquired about has two scenarios: First, if the manager is authorized to increase or decrease the cement sales, then there is no harm in increasing or mediating, and taking a fee for mediation is subject to the issue of the "price of influence" (thaman al-jah). Second, if the manager is prohibited from increasing, then neither increasing nor mediating in it is permissible, and taking a fee for this mediation is considered a forbidden bribe. The Messenger of Allah, peace and blessings be upon him, cursed the briber, the bribed, and the mediator.