Your problem here is that you are breaching the rules as to how much much RMB you are allowed to take out of China.

Fine if you dont; get caught. Less than fine if you do.

As an FT (Foreign Teacher ) in China you are, assuming legal employment, permited to exchange 70% (generally) of your earnings. I presume you have done this if if possible.

Additionally many banks will change money on an illegal basis. I have less than fond memories of spending five hours explaining to my local bank the regulations as to how much I could legally change, without any success. And then they offer to change as much as I want on a sans receipt basis when I had admitted defeat.