Advocate not returning my case file

No. There is no rule of this nature. 

There is no such rule of taking thump impression or signature from the client, but it is there that you can obtain a NO DUE CERTIFICATE, from your advocate as regard the fee relating to the case. It may be that your advocate must be in the apprehension of the file being lost in transmission through courier. Ask your advocate to sent a certificate as regard no due of your particular case. After getting the certificate ask him to send it by most reliable courier service or make a personal visit. . In spite of the above the advocates retains the file, approach the Bar counsel office of Maharastra with a complaint mentioning all details.

You have not mentioned in whose favour the case is decided, if it has gone against you, then there is a time limit to approach the higher court, do it urgently.

Dear Sir,

Acknowledgement of case file is necessary ofcourse not by way of Thumb impression and with out clearance certificate from the present Advocate another Advocate may not take up the case.  If there is any more pinch, you may approach the local President and Secretary of Bar Association.

Regards,

MSR.Murty