Is it permissible for someone who has received a building permit, which allows them to reserve 3 or 4 cement trucks from the factory, to sell the surplus quantity and benefit from its price, knowing that the price of cement from the factory is much cheaper than its price in the market?

Whoever owns something through a sound and legitimate ownership is permitted to dispose of it as they wish, whether by selling, gifting, or lending. The seller does not have the right to prevent the buyer from disposing of the sold item. If such a condition is stipulated, the sale is originally invalid. It has also been said: the sale is valid, and the condition is void. Therefore, you are permitted to sell any trucks exceeding your needs without reservation.