You've opened a can of worms!! If money was like pure energy then yes, country A getting wealthier must affect B which loses out proportionally because the total money in that system remains the same. BUT I don't think this conservation law holds water when money is involved!
I presume you mean that country A is currently China and that B= rest of the world!!
As a kid I used to puzzle about the UK's Royal Mint in London, which printed and made all the money for the UK. If money is just money then why not just print more when the going gets tough!! I didn't realise that the amount of money you can physically create is related to the amount of gold in the reserve? Is this correct. Is it the same for all countries?
I'm still puzzled by this idea!! If you know the answer I'd appreciate it.
In the recent banking crisis and recession not one expert economist predicted such a fall - all they talked about was bubbles bursting and markets being nervous- they don't have accurate forecasts!