query against clause in offer letter

I have received a offer letter from company.it is mention that is I don't join after accepting offer I have to pay 20% of CTC to company.i have not signed any letter with company.i have just given confirmation on mail.can I back out from my decision as my current employer is not reliving me.

I have just mail that I accept and will join by joining dat.I haven't signed any doc which they are continuously demanding