First speak with the company where you got your offer from and explain them the situation. Have all the communication marked to your personal email. You have been working there for the last 5 months, so not to be worried about. Please approach a corporate attorney and seek his help. You are not liable to pay the entire salary and they cannot withheld an employee's salary for the period he had worked for.
Only catch is that you need to convince your new employer on the situation and explain them that your present employer is threatening you and your salary has been withheld.
14th October 2016 From India, Bangalore
You are not legally bound to return salaries paid and moreover you are entitled for complete settlement till the time you serve in your current organisation. If they are threatening you, ask them to give things in writing and moreover you can threaten them back that you will take it up legally. Serve them a legal notice, if required, it would not cost you much, you might have to just approach a lawyer to draft the notice.
At the same time, you can tell the new company you want to join that you would not be able to provide relieving letter/ service certificate etc. and instead of giving them a months notice, you can send them an email stating that as my salary, dues are not paid and F&F is not getting processed, I am leaving the organisation. State in an email that they have threatened you and hence you are forced to abandon the company. I do not think they can do anything.
Keep everything documented so that you can you can produce it in case of any legal proceedings at a later date.
15th October 2016 From India, Pune