Does the absence of a passenger, with whom a monthly fee has been agreed upon for round-trip transportation, reduce the agreed-upon amount?

If the two parties agree on delivering the liquid on specific days and at specific times, then he is considered a special employee (ajīr khāṣṣ) during these times, and he is entitled to the full wage whether work is available or not. This is because by surrendering himself for the agreed-upon period, his services become due to the hirer, and the wage is in exchange for these services.