If they have not been able to provide you with the documents then they dont have proof of this. By law, they (collection agency) are supposed to provide you with all necessary proof indicating that you owe this money. You need to send them a Certified letter asking them for this proof and they have 30 days to respond. Make sure that you get proof of delivery from the post office, so you can show this to the Credit Bureau if they fail to answer within a reasonable amount of time. if they do not, then by law they have to stop harassing you and remove the negative mark from your credit rating.