如何限制股東公司的股份變動?

一間公司的基本權益在於公司組織章程內。按照你的問題，ABC應該唔係做XYZ嘅大股東，咁你地可以做股東協議書，由XYZ現有股東同ABC簽訂。由於每一間公司情況都會唔同，我都係建議你唔好慳：上律師事務所做合約會安心啲。一般股份轉讓要在公司董事局會議中通過﹝ABC自己要先開會同意入股XYZ，而XYZ要同意由現有股東轉讓股份俾ABC﹞，而轉讓者要先俾XYZ現有股東出價承讓，然後至可以攞出街賣。
希望幫到你！

If the company is a private limited company, the transfer of shares is restricted. Under the pre-emptive clause, XYZ Ltd should give the existing shareholders a priority to buy back the shares before he could offer it to outsider. Moreover, the directors have the right to decline to register such transfer. Anyway, check with your Articles of Association of the company, there must be such a clause in it under the heading "Transfer of Shares". The ultimate power vested with the % of your holding in the ABC Ltd.
If you really to have an agreement be executed, this should be signed by all shareholders of ABC Ltd. under the "Shareholders' Agreement". As this involved the intention of all parties, I recommend it may be better to have this be done via lawyers