Is taking one-third of the truffle harvest permissible or forbidden, knowing that I own the car, transport the workers, and provide them with some services?

If a group agrees to jointly gather a permissible item and divide the yield among themselves, such as gathering truffles, this is called "Sharikat al-Abdan" (partnership of bodies/labor) and is permitted by the Maliki and Hanbali schools of thought.

One form of this is for the partners to work with their own hands, or for one of them to contribute with a tool or a workplace, and the wages between them are equally divided or differentiated according to the agreement.

The requirement of justice in an absolute partnership is equality in work and wages. If one of them works more, he is either donating his extra effort, in which case he receives the same wages as them, or he demands an increase in wages, or an increase is agreed upon for him.

If the owner of the car participated in the work and transported them with his car, it is just for his share of the profit to increase due to his participation with additional work, and this is permissible.

If he only transported them with his car and did not participate in the gathering, then the agreed-upon percentage is considered a rental fee for his car, and rental agreements are based on mutual consent.