Co-signer question.....?

Can a debt collector demand full payment (today) from a co-signer? We made an arrangement to pay1800 then $100 a month for 6 months at which we could "make a pay-off deal" or continue with payments. The 6 months is here and now the firm says that the person that use to work for them shouldn't have made that deal and the 6 months is up and they are demanding that to co-signer pay 2500... today! They sent a letter stating that our payment arrangement has been canceled due to non payment!!! We've been giving them 100 dollars every month faithfully to get the debt paid off and now all of a sudden there is interest that we never knew before that costs us 110 a month so we are falling behind and weren't aware of that until today! Any advice on this? I would like to continue to pay this off but they are demanding that my parents (the co-signers) put it on their credit card today!!!! What should we do? There have been no late payments in 6 months and unfortunatley nothing in writing about our agreement! They are threatening wage garnishment and leins on my home and my parents! Is there some law that says I have to pay it in full? It's with Alan M Laskin (CACH, LLC) if anyone is familiar with them.....any advice would be GREATLY appreciated!