Did you have the agreement regarding the settlement of the debt in writing? If so you should have taken that to court (or indeed, just presented it to them), if

Did you have the agreement regarding the settlement of the debt in writing? If so you should have taken that to court (or indeed, just presented it to them), if not then that was your problem. Indeed, exactly the same thing could have happened if you'd paid the debt in cash with no record of the transaction, whereas a recorded bank transfer would be hard for them to conveniently forget about.