Contract Law practice question

Tim lost his new pet dog, a black-and-white Shetland sheepdog. He put up notices around his estate stating that he would give a reward of $100 to anyone who finds and returns his pet to him. A few years passed by with no results. Tim then just gave up hope that he would find his pet again. After 7 years, Jim brings a haggard looking black-and-white Shetland sheepdog to Tim and claims the reward.

Applying and citing the relevant principles of contract law, explain whether Tim was obliged to pay the reward.