The DD would have been paid by a cheque of your bank a/c and using all that details can be obtained.
Kindly put the legal notice wordings on the forum(mask names etc) so that legal experts on forum can advice.
Check with courier company details of delivery.
It is not clear what else you were expected to do,if as you state the demanded amount has been paid.
Details need to be given for better understanding.
Most important speak to a local lawyer in your place.
Very casual on your part and this is what has ut you in a situation of facing legal proceedings.
But what I cannot make out is after payment why has the company gone legal??
Any other terms and conditions which have been breached ??
The case pertains to May 2012 and now matter is going to be legal-why after 3.6 years?
What is the history or cause of legal proceedings being launched so late??
When you paid, your intention was good. But the employer seems to be a complicated one. You have also been casual about it in as much as you have not closed the loose ends here. I wish you good luck for your legal battle and also for future.