Sallie Mae Drama...can they do this?

2 years ago I sent to school & Sallie Mae was the only lender. After graduating I was only able to find one job working 8 hours per week. I tried to work out monthly payment arrangements with them, but it wasn't enough for them so they sent me to collections. The collections company called & demanded the whole $8,000. Naturally I didn't have it. They even researched me, found out I'm married, checked my husband's credit without our authorization, & saw that he has enough credit to put the $8k on his credit cards. We can't afford a higher credit card payment, so of course I said no. So they told me I would be going to court.

They called back saying they would settle the $8k debt for half, which is $4k, and to avoid court, I had one hour to call back with arrangements to pay that day. Frantic I called my parents, & they took the money out of their equity, & that $4k was paid that day. The collections agency then faxed back a letter saying that the $8k amount was settled for $4k, and the BALANCE IS ZERO.

That was like 3 months ago, & I thought Sallie Mae & that collections agency was history. This morning I got a call from Sallie Mae saying I still owe that other $4k. WHAT?!?!

I still have that letter from the collections company saying the debt was settled & the balance is zero. Can Sallie Mae still make me pay the other $4k???