Katherine lost her diamond ring while shopping in the local supermarket. She put up a reward notice outside the store in these terms, "Reward of $200 for the return of diamond ring lost in this shop on 1 September 2021. Call this number to claim the reward 04041113345." Halima finds the ring in the dairy section of the shop on 3 September and takes it to the Service Desk. She hands the ring to Ali, who is an employee of the supermarket and is working at the Service Desk on that day. Halima does not know about the reward notice. Ali knows about the reward notice and calls the number to claim the reward. Is there a valid contract in these circumstances? Can Ali enforce his right to receive the $200 reward?