Commercial contracts does not work like that. For acquisition contracts like that, it is likely that the final payment - which tends to be a significant portion of the full value - is required to be paid on delivery of the product.
Now I do not know the detail, but my speculation is the debate around what constitutes the term "delivery". I guess one party is trying to withhold payment by arguing that with the buses not being licensed into service, they would not be count as "delivered" and hence they are not liable to make the final payment, whilst the other argues that the vehicles have been taken up at the other's permises, so as far as they are concerned, they have been "delivered".
By common sense, it's probably not difficult to see what the issue is. However, in the commercial or legal world, this is far less clear cut. But the bottom line is, for HK you have one (or a pair) of operators who are very close to its key competitor, whilst the only other customer is being so cheeky. So it should not come as a surprise to see someone saying enough is enough and pull out - at least on a short term basis.
If it sounds extreme, then look no further than Mercedes, who has excellent presents in Europe as well as certain parts of Asia. Despite HK being a substantial market, if a manufacturer sees that the effort to break into it is not worthy, then they won't bother. It is business world, afterall, and it is all about making money.
Probably have to bear in mind that exchange rates come into play as well. My gathering was that when KMB ordered these B9s, price was one of the factors driven by favourable exchange rates.
And of course, we all know exchange rate fluctuates and that alone can upset the price on the table significantly... What it was years back is rather different from what it is say over the last year.
... 
... 
