Please type the answer by the computer, so I can see it clearly, thank you!!!! (Hong Kong Business Law)

Since high school, Tom and Ann have been great friends. Ann works as an accounting manager at an insurance company, while Tom runs a home-based office carpet cleaning service.

Ann introduces Tom to Tony, the managing director of EFG Ltd, at a social function. Tony agrees to utilize Tom's services for three years, beginning in November, at the dinner table. On the drive home, Tom is ecstatic to have signed the new contract; he pledges to send Ann $5,000 as a token of his gratitude, but never does so.

Tom also has a five-year contract to clean the carpets at Wong & Wai Primary School, which pays him $10,000 a month. Many of his clients started to cancel their orders as a result of the severe competition. Because Tom does not want to lose another customer, he tells Wong & Wai that he is willing to accept $9,000 per month for carpet cleaning instead of $10,000 per month. Wong & Wai enthusiastically agree.

Tom writes to Wong & Wai eight months later, requesting that the school refund him $8,000, the difference between the initial contract price and the lowered price for the services he provided in the previous eight months. He also intends to charge the initial monthly fee of $10,000 from now on. His plea is turned down by Wong & Wai.

Question:

Hong Kong Business Law, what is the application of the law in this case? (Please explain in detail why this application, Can be more than one application (300 words))