Sallie Mae Taking My 3rd Party Payments But It Is Not Reflecting On My Statements

Hey guys, so my job offers tuition forgiveness. My due date is on the 22nd every month. My payment that was due on 2-22-25 was wrong. I was overcharged by roughly $150. I called Sallie Mae on my way to work. Sallie Mae gave me every possible excuse OTHER than that they were wrong and overcharged me. They claimed that they never received a 3rd party payment of $416.67. They stated that they received the 1-21-25 3rd party payment which was applied to the 1-22-25 payment. I messaged my 3rd party payer for more information after getting off the phone with Sallie Mae. I’m currently waiting to hear back from them. On the Sallie Mae website I found the payment that they forgot to calculate into my 2-22-25 payment. The 3rd party payment was posted on 1-29-25.

I will be calling Sallie Mae again in the AM. What else can I do to ensure that the overpayment is either refunded or applied to the principal?

Additionally what is a good method to document payments made/ received etc.? In the event that this continues, and Sallie Mae refuses to acknowledge payments and continue to over charge me, would I need to sue in order for payments to be acknowledge?