Contract Law Advice

Valdima

Just got a couple of questions to do with contract law


John is a keen soccer enthusiast. On 31st July, he joined his local soccer club. On 20th August whilst visiting the club he lost his TAG Heuer gold watch. He decided to put an advertisement on the noticeboard of the club which read:
"Help required:
Lost TAG Heuer gold watch.
Reward of $300 to anyone who can return the watch or has information relating to its safe return.
Ring John on 0412 345 567"

On 20th August Sean is walking down the street when he noticed the TAG Heuer gold watch on the floor. He picked it up and took it to the local police station where he handed it in.
Sally is a member of the soccer club. She sees John's advertisement and thinks nothing more about it until she is talking with her next door neighbour Sean on 6th September.
Sean informs her that he found a TAG Heuer gold watch and handed it in to the police. After concluding their conversations Sally immediately rings John and informs him of the location of the watch. Sally asked for the money in the advertisement as she had completed what she thought she had to. John thanked Sally for the information and informed her that he did not consider himself bound by the advertisement as his insurance had indemnified him for the lost watch and so he did not care anymore about the location of the watch.

What are Johns Liabilities ?