Sallie Mae Sallie Mae Sold a Fully Paid Account Back to the Dept of Ed Nationwide

The Department of Education student loan was for some secretarial schooling.  Due to the household finances, we were unable to make the monthly payments, and the US DEpt of Ed debt was turned over to Sallie Mae.  Sallie Mae had the debt when the next letter arrived.

My husband and I could not pay the student loan, so my father and mother paid off the debt, in a settlement.  The settlement was for a lump sum, excluding the interest, or something similar to that.  The settlement stated that if the settlement amount was paid in full, the account would be considered "Paid in Full".  The check was written by my Dad, and sent to Sllie Mae.

Many years later, around 2012, the Dept of Ed calls to collect this debt.  Sallie Mae sold a fully paid debt...back to the Dept of Ed. What am I to do, if I cannot find the letter of "Paid in Full"...if it even exists?