Background Information: Lynn is Andrew's wife. Last Friday, she attended a party where a lot of movie stars participated. Before she went to the party, she visited Mary's hairdressing salon. After Mary finished setting her hair, she was so pleased with what Mary had done and said that she would give her an extra HK$3,000. Because she had not brought enough money with her, Lynn said that she would return the next Monday to give the extra money. After the party, Lynn changed her mind and did not want to pay the HK$3,000.

Question: Lynn finally did not pay the HK$3,000 to Mary. Mary wants to take legal action to obtain the sum. Advise Mary the likelihood of success of her claim. Support your answer with detailed explanation of the relevant legal principles and cases.