Possible Rip Off

I am trying to find out what rights I have pertaing to settling a debt in full with Allstate Financial Services. I am overseas and dealt with on e of their collection reps. We agreed on a settlement of %50 plus their processing fee. I paid the settlement in full. ONly problem is they will not send me a letter showing I paid this in full. As a matter of fact they are still reporting me as having a late balance. I don't know what to do. Please give me advise as to how to handle this. Everything was negotiated and agreed on by telephone.

Thank you