is it illegal for a collection company to fake a settlement aggrement that you pay, but suddenly they sell or transfer your account before it settled?

A company called FMS Investment Corp. had my debt in their office. They were owned by Navient the current holder of my student loan debt. FMS set up a payment arrangement with me or an agreed amount or settlement, and told me that a paper would be sent as validation to the arrangements for 2300 more or less ( not sure of precise amount). Payments were set up and payed I fulfilled my part of the verbal agreement. However before the last two settlement payments were made my account was transferred to Navient now they handle my account in full and will not honor the payment arrangement set up with FMS the company Navient own(ed). I have bank statements confirming that I fulfilled my obligations on paying of agreed to payments and paying on time. However I feel like I am being scammed. The agreement was set up under the guise of a settlement and FMS or Navient will not work with me to resolve the issue Please help me in understanding what my legal options may be Thank you.