What are the differences and boundaries between a contract of جعالة (reward-based contract) and a contract of إجارة (lease/hire contract) for a specific work, and what is the ruling on not completing the agreed-upon work in each of them?

Scholars have differentiated between ijarah (leasing/hiring) and ju'alah (commission/reward) based on three fundamental distinctions: whether the contract is binding or non-binding, whether the work (its duration and quantity) is known or unknown, and whether the worker is specified or unspecified. Ijarah is a binding and known contract, whereas ju'alah is a non-binding contract, and the work within it may be unknown or involve gharar (uncertainty). Some transactions may oscillate between ju'alah and ijarah, such as a doctor's condition for curing a patient or a teacher's condition for teaching the Quran.