Help with NES

Hi I need a little advice on dealing with two collection agencies. This is a Salli Mae account that I co-signed for, well it went into default and I settled with a agency in May of this year for 8k. Last week I received a call from another company saying that I still owe the remaining balance of $4K. I was so caught off guard by this that I dont remember ever receiving a payoff letter from NES. I spoke to then on Friday and they told me that there computers say that they offered me the settlement and that I excepted it but they needed to contact Salli Mae. I know I should have gotten a letter right off the bat but I was so worried about losing everything that I was not thinking straight. What can I do now??
Thanks