有關Law的問題一問

Can't type Chinese on the machine so cut & paste only.

他究竟是否已違約 - No
Reason:
In English, it is called Frustration. I will skip the technical details. In short, something happened making it impossible for a party to continue with the contract.
用《法律修訂及改革(綜合)條例》23章16條的語言,Frustration就是
"合約變成不可能履行或因其他原因而受挫失效而合約各方進一步履行該合約的責任亦因此解除"

未交付的餘款 - same as above "各方進一步履行該合約的責任亦因此解除"

已交付的定金 - 《法律修訂及改革(綜合)條例》23章16條(2)
在合約各方責任如此解除的時間前依據該合約已支付...予任何一方的所有款項，須(就如此已支付的款項)作為收款一方收取以供付款一方使用的金錢而向收款一方追討...：
但如已收款...的一方於解約時間前，在履行合約時或為了履行合約而招致開支，則法院經顧及案件的所有情況後，如認為容許已收款的一方保留全部或部分已支付的款項...是公正的，則法院可如此行事。
It can be difficult to understand the meaning here. Simple answer is 招小姐可向吳先生追討款項,如吳先生於解約前為了履行合約而招致開支,法院可容許吳先生保留全部或部分款項. But he has to thing about the cost of going to court for < $250.

雙方合約仍然有效,只係交付期將伸展至無限期.

情況等同扣關,唔通買賣方可以因此而違約?

1. No. The contract is void as it is against public policy (no privately owned bicycle).

2. The money will be refunded and the contract is voided (not breached).