my ex employer paid salary by mistake

What is the legal issue involved in the facts vis-a-vis query for consideration and obligation ?

For a wrong credit into your account, your ex-employer cannot prosecute you by filing an FIR etc. It is their fault and not yours and moreover, you are ready to send the amount back.  The bank shall not reverse the entry based on the request of the sender. 

You as the Account Holder, need to inform the bank about the wrong credit into your account with full details of amount, date, etc, and request them to reverse the entry back to the remitter from whom it was received. Bank may seek your request in writing and you may give it after ensuring that the credit does not pertain to you.

You should address an official letter to your ex-employer setting out the details of erroneous credit to your account and offer to reimburse with your cheque in his favour, and issue the cheque in the name of your ex-empoyer on receiving his confirmation.

The ex-employer may through their banker request you to recredit the amount after obtaining NOC from you.  Normally as per SC Judgment, once the credit was made, the employer can not seek the bank to debit such account, and account holder consent is mandatory..  A simple matter that can be solved through your banker leading to ego issues and unnecessary apprehensions.  If you are aware that there was a wrong credit, ask your bank to debit your account and send the amount to the bank that sent the amount. (your fears/Apprehensions may be sound with individuals but not with institutions, your ex-employer.

This is a simple problem. You are worried only about the authenticity of the ex-employer's bank account. You ask the ex-employer to request you in writing giving his name and bank account details. After that you transfer the amount. Once he gives you in writing, it is for him to worry about authenticity. The bank cannot transfer funds from your account without your asking for it.