Eek, as a general practice, I would suggest never letting your boss keep your Foreign Expert Certificate.
However, I'm not certain that is actually the "certificate" they are looking for. I believe you require what essentially amounts to a receipt for the amount you've earned. If everything with your work is legit, they'll be able to produce this for you without too much hassle. If they've fudged your visa (got you an F not the proper Z) and are technically having you work "under the table" and in tern are avoiding paying tax on your earnings, this might not be possible to get.
Either way - like most things in this country - it's just easier (and often expected) that you'll just go around the "system". Like Brendan said, try to find a local that can help ya, or just hunt down a black market currency trader.
Re-reading your OP, i'm not 100% though what you're trying to do. Are you trying to switch GBP or USD into CNY? or CNY into a foreign currency?
BTW: Good to have ya on here - and yeah, I hope it grows too

If slow and steady wins the race, we're up by aces.