disputing debt when faced with being sued

I was just informed that my cosigner was notified of an attempt to collect a debt that I don't believe I owe. She was told that she had 21 days to pay or she was being sued. Of course this puts me in a tough position. Is there any way I can stop these proceedings without admitting I owe the debt. They claim I owe $2000 on an origonal debt of $1000 and it has been six years since the origonal debt. The statute of limitations is 6 years in my state and it's about to expire. I have not kept the reciepts because it's been so long but I can get the cancelled checks from the bank for a fee. Even if these don't add up to the $1000 because I paid some of them with certified check does it still prove that there claim is wrong and thus invalid? Do I have any options here besides just paying money I don't owe so my cosigner doesn't have to be put in this position? Will most collection agencys negotiate a lower amount or agree to a payment plan if I decide to go ahead and pay it to avoid the hassle. I don't have the money since I lost my job a while back. Please help