Question on bills going to a collection agency?

So I have an $800 and some dollar dr bill that went into collections. I guess I wasn't paying enough on it a month.... Blahhh... Anyways, the collection agency called me a few weeks ago wanting to set up automatic payments on my bill. I did... I agreed to pay $140/month till its paid for.(then I had NO money left over with that new bill and other bills) Now, I got moved to a different area at work making $4 LESS than what I made before. Can I can them and tell them I need my payments lowered? Or am I SOL? I cannot afford that right now with my new wage. :-/