Corporate Receivables Inc. had agreement to settle debt, now they say I didnt phoenix, Arizona

Had agreement to settle debt for $1200 paid $800 up front then they agreed to take out payments of $100 for next 4 months.  John Miller was the man I talked to,  very nice man. Final month was paid I got a letter saying I owed $828 I call to find out whats going on.  I talked to a man named Joe Owens he was very rude.  He informed me Miller no longer works there and I have to pay the remaining amount.  I even talked to the security center to verify it but didnt get anything in writing.