And by the way, whatever they may ask, the ONLY thing you need is your passport, a visa and of course your yellow card. All the other things they ask (invitation letter, ...) is utter and complete bullshit.
The Congolese authorities will literally try everything - I have known cases where they tried asking for a visa for a dog, where they said that 3 free pages in the passport was not sufficient, where they claimed a yellow card had to be in French - if you can make it up, the Congolese have tried it before you. Just smile, insist, if needed get other people involved, show that you are sure and not afraid (utterly important that one, they can SMELL fear), and make sure the official loses his time with you so he can't get other bribes while he is dealing with you. One note however: be careful to get higher authorities involved, this can often only make things more of a problem (read: more expensive) to solve. Rather get other "clients" involved - they will tend to sympathise with you if they are local, or they might be able to help you if they are "muzungu".
Ah, how I love Congo - but how fecking happy I will be to leave the country in a few weeks...