Are you assuming China's not paying in dollars, or do you know this?
Lula: Let's trade in our own currencies
China and Brazil: Dump the Dollar
But there's also been a bit of backpedaling, or soft-soaping, however you want to put it:
Brazil, China Plan to Trade Without Dollars Is Pure Idle Talk
China, Brazil Yuan Trade Will Take Years, Bank of China Says
Guess they got scared of setting off a dollar-dumping frenzy too early...