Certegy Check HELP!!!!

I wrote a check in December 2009 to the Mattress Firm for $2000. An issue with some charges being put through earlier than expected put me back a few hundred dollars. I lost my job and got a new one 2 weeks ago. So i called Certegy and told them I could make a $100 payment on Friday (2 days away), and also wanted to set up $100 payments every week until paid off. That's $400 a month. They told me that unless I could set of payments of $500 a month they would not accept my payment offer, but that I could go online or mail in a money order for however much I wanted to. But if I did not pay the amount they wanted me to then it would not be part of the payment plan, therefore my check could be sent out "for further collection". So my question is this: I am making an effort to resolve this debt, what kind of action can they pursue if I am making these payments? Why wont they accept my offer, arn't they aware that not everyone can afford $500 a month?

I do understand that I did make a mistake, but how can i make this right now?