Abscond Company Due To Wrong Work Assigned With Withdrawal Of One Month Salary

Hi,
Join a company one month ago they ask me to work different technology rather than for which i joined also asked to work on support project which is already developed but i joined as developer so not feel comfortable.
Due to this reason i search new job and Abscond.
HR mail me to ask for reason so i replied that i got new opportunity so i left company instead of saying real reason i.e.( due to work work assigned).
Company send me mail that they want one month salary back or they will send legal notice to my current and old organization.
need a advice for the same.
what to do?
Please let me know a link where a clause which help me to resolve the same.
Or should i pay them if they are ok to give an releiving letter?

Dear Chethan,
It is my personal opinion that you should return that one month salary and finish off the matter. Obtain proper relieving letter and "No Due Certificate".
Imagine if you do not do this then what will happen? Possibly company may sue you. Are you prepared for litigation? Company has got resources at its disposal to fight even frivolous case of this kind. But can you do that? What will happen if your employer comes to know about your questionable past?
If you got better job then it is important to concentrate at the current job rather than tying some lumber to your feet.
Ok...
Dinesh V Divekar

Hi dinesh sir,
1. the appointment me as dotnet developer
2. When i join they assign me excel work as well as some support projects in dotnet but i'm developer
3.When HR ask reason i replied that due to onsite opportunity insted of saying the truth that wrong work assigned to me.
?) does i'm liable to return full payment or should i return only basic salary?

Dear Chethan,
Whatever was the reason, today's truth is that you have left them and working elsewhere. So why not to finish the matter even with complete return of salary?
While leaving the job, why you did not reveal the truth? HR could have assigned you the job for which you were selected?
It appears that you are little reluctant to return the salary. Talk to HR and find out whether they are ready to accept the partial return of salary. If they accept the partial return well and good.
Lastly, please compare benefits of returning salary Vs benefits of not returning the salary. Choose option that you suits you most.
Ok...
Dinesh V Divekar

Dear Dinesh ,
i'm ready to return the payment.
1. what should be the mode of payment by Cheque/ online/DD?
2. What kind of documents i should expect from them
a. Releaving / experiance letter.
b. NOC certificate.
Thanks
Regards,
Chethan