I got an invitation letter from a chinese company for a business trip. However it was not duly sign

by an authourise body in China, so I requested that the letter be taken to the relivant authourity for signature, They aggreed but complain that a charge of $500 is required by the authourity which they wish I should assist in the payment. Should I go a head to pay for it? Its my first attempt in doing business with them. Please advice me on what to do. Again, is there a chineese body one can seek invitation letter through in Nigeria or China?

Thank you,

Mr. Egoro