Collector sent me PAID IN FULL letter by mistake..now trying to collect

by Jesse
(West Covina)

I made an agreement with my debtor to make payments for 8 months till debt is paid. After only 3 payments I received a formal letter from this credit bureau that my account settlement has been "settled" and is "Paid in full" and that no money is no longer due on this account. I know it was not...I though they must of accepted a lower amount and settled. 2 months went by and now they are calling me saying that they sent the letter by mistake and that I am 2 months behind and that I must pay everything now. They are being very rude and obstinate. Do I have to pay? or is this document legal? will it hold up in court?