As you said that you have received the appointment letter and in that there is a clause that there is no need to give one month notice until the job is confirmed / you receive confirmation letter then you can resigned any time. But still they deduct the one month salary then it is illegal. You can request once again them and try to solve through verbally first and still there is no result then you can approach the advocate but if the amount is not on higher side better to leave that option.
If your matter is not solve verbally then warn them that you will approach to labour court in case your notice period amount is not refunded. Might they refund your money.
Agriya HR Solutions