hello, I am a Nigerian

Ugo, your official invitation letter is enough for the application, no need to get another official invitation letter then.

Hello, I was told i still need an official invitation letter from the Chinese Embassy, because Nigerian embassy doesn't issue a tourist visa, and to get a business M type i will need the letter... please is there anyone you can refer me to, so as to handle the application personally?

If so, you can try to apply for a business M type for that. You can ask the embassy that if the invitation letter from a duly authorized unit and the invitation letter from the business partner are both required for that.