What you are doing is called “Settled for less than full amount or full balance”. First, contact them and get them to send IN WRITING the full amount they will accept. After you get it IN WRITING, go to the bank, get a cashier’s check and copy the check and the letter. Mail the payment to them via Priority Mail with Signature required. You want to track that they got it and make someone sign for it.
Documentation is the only thing you will have on your side.
Pay nothing until you get it in writing. Keep copies of everything.