It is really difficult to change money in China. Most banks insist on seeing a tax receipt to prove that you have earned the money legally. 
Some of us don’t have tax receipts so have to find the banks which do not ask for them. It is hard. And when you find a bank (eg ICBC bank) the queues are long long long (37 people in the queue was my record).
If you can’t be bothered to wait you can go to the Bank of China in the city centre and there you will find two men (sometimes three) who work as a pack. They offer a quick and easy currency exchange service. All you need to do is sidle up to one of them and wave a bunch of notes at them. I tell him I want dollars, he looks up at the current exchange rate shown on the Bank of China rate board, offers a better rate, we haggle then agree.
He then jumps the queue and goes straight to a counter (even if there is someone already there), gets his bank book out and the girl gives him some dollars. He walks back with a fistful of dollars and we exchange wads. It takes less than 5 minutes, there’s no need for tax receipts, no queuing and the rate is better than the bank’s.
The bank has lots of staff in the lobby milling about waiting to help. In addition there are bored uniformed security guards playing with their rubber batons. The dodgy dealers sit on lovely soft office chairs at the bank’s desks and take business from the bank. I asked one of the bank’s staff once why they tolerated these men sitting in their bank. She said “It’s not illegal, we cannot stop them”
I for one, enjoy this service. Long may it remain.