What is the ruling on someone who brings inexpensive medicine to a sick person, and the sick person offers him a car or a house in exchange for bringing it?

A man's statement: "Whoever brings me the medicine for this disease will have this car or this house" is considered a ju'ala (reward-based) contract. Its validity is conditioned upon knowledge of the compensation. If the car or house is specified with a description that eliminates ambiguity, then whoever brings the medicine is entitled to the reward. The man is permitted to revoke the contract before the medicine is brought, and nothing is binding upon him.