shanghai police southern china detain seven people connection underground bank scheme involve 20 billion yuan billion state news agency xinhua report suspicious bank account shaoguan city guangdong province investigation snowball involve suspect 10,000 people 148 account across 20 provinces xinhua report suspect allegedly profit change exchange rat yuan hong kong dollars say without give detail yuan renminbi fully convertible government limit amount foreign currency individuals businesses china access give rise network underground money changers bank 6.6086 chinese yuan renminbi chinese police detain seven multi-billion underground currency scheme