Exececutives or idiots?

Hi!
3 months ago my project manager handed me over a source document to be computerized, I did it succesfully. In the meanwhile he left the firm, afterwards when we went to target site with finished product to fetch paycheck, it came to our knowledge that we have to develope lot of other things according to the agreement, signed by our Project man, damn it. We succesfully received half of the payment on promise to develope rest of the software. But you see it is hell lot of tension.
What was in mind of PM while signing the contract, I can't understand it.
Moreover my boss is pushing me to deal with the management concerns, I explained him that dealing with client is none of my business but he's still using me to deliver the receipt to customer, and says me "you know it better so you will deal it." Previous visit of my office mate with boss have also spoiled the matter. Client says they explained the new source document to Boss and another programmer, whereas they have nearly no idea of source. Today when I went to client and asked him to explain me the document he simply said I've done it before. Some times it becomes a problem to tackle difficulties as it is now, but I know its only me who will help me out of this and any other difficulty in my life. May God help me.
Bye