Ruling 2243
If the offeror wishes to annul the reward after the worker has started to perform the task, it is problematic unless he and the worker come to an agreement.
-Ayatullah Sistani, Practical Laws of Islam, Reward (Juʿalah)
Reward: Annulling a Reward After Starting the Task
Practical Laws of Islam as per the teachings of Ayatullah Sistani
