thank you both so much, I'm now on with it

Hi All, Just a bit of advice needed for my boyfriend. I don't have all the information but I will tell you all I know and get any info off him that I need for you to help me. He made an agreement for car finance with first response finance (don't know the date but he says this has been going on since 2009). He sold the car earlier this year after he was fully satisfied that all payments were made. A HPI check before selling this on was clear. Since then, First Response have been hassling him saying that there is still an amount owed and he has committed "Conversion Fraud" because he sold it on with finance outstanding???? They have said "On the 20th January 2015 First Response Finance Limited issued a vehicle deletion correspondence to the correct agencies to advise that our interest has been removed from the vehicle. This is a business decision based on individual factors and is the reason the vehicle no longer shows as having a financial interest marker". He is sure he has paid it off. They have said they have tried to contact him about it lots of times before the letter he has actually received and have been quite nasty with their response, saying for him to contact them to arrange a payment schedule. How do we proceed with this? Do we send a prove it letter or deal with it in some other way? Hes worried that he will be made to pay this one way or another and doesn't have the funds to do so. Whats the best way forward?