What is the ruling of Islamic law on selling trucks that are not owned by the seller and for which the full price is not paid in advance, but rather by agreement with the customer to pay half the price in advance and the remainder upon the truck's arrival? And is the commission earned from this type of sale permissible or forbidden?

It is permissible for the establishment to act as a broker between the client and the truck owner, provided the client is informed of this and a commission is taken. It is also permissible for the client to ask the establishment to purchase a truck with specific specifications and promise to buy it from them at a profit, on condition that the establishment first buys the truck and then sells it to the client with an independent contract. However, selling the truck to the client before owning it is not permissible due to the Prophet's prohibition against selling what one does not own. If the transaction was prohibited and you were unaware of its prohibition, there is no sin in the commission earned, provided you repent and do not return to it. If you have doubts about a matter, you should stop the action until you are sure of its permissibility.