Asking more money even after settlement

Location/place: Thane, Mumbai

Name of company/service: HSBC Bank,VASHI

Dear Sir,

I have settled my HSBC Credit card in 2010 after paying Rs. 10000.Now after 03 years this people are demanding more 5000 from me.They are telling me it should have been done in one installment but it is not written in letter.I have receipt of 5000,3000 & 2000.They have told in the letter that I should pay all dues before 28th February which I have done.Please direct them to issue one No DUE CERTIFICATE to me and pay me rs 20000 from harassment.