Your liability exists till the mortgage of property is in operation. If you can prove that you have given the cheques in lieu of that mortgage and the mortgage is now cancelled, it will automatically exit you from any liability.
For your benefit, must do the follows.
1. File a criminal complaint against the cheque holder describing all the facts (for not returning the cheques, which are lying with him as security and making misuse of the cheques).
2. Along with copy of FIR, go to the bank and tell them to stop the payment of cheques, as somebody is trying to blackmail you.
At last, use the word "security cheques" instead of only word "cheque" every where you have to discuss this matter.
The term of "Security cheques" keep you out from the sec-138 of Negotiable Instrument Act (criminal liability) and you will remain liable for civil suit (recovery for money) only.