1. You will have to apply to the High Court for grant of Probate for your mother's Will. The fixed court fees for the same is Rs. 75k plus lawyer fees plus clerkage.
2. Your buyer may not agree to buy the property from you since there is no Probate granted for your mother's Will.
3. However if it is agreeable to your buyer, then you can propose him the following course to avoid incurring expenses for obtaining Probate from court:
a. issue a public notice in newspapers informing the public at large that he intends to buy the property from you. That the property has devolved on you by your mother's registered Will. That if any members of public have any objections against this transaction, then to submit the said objections within 14 days of the publishing of public notice
b. you can give him an indemnity bond stating that if in future any person claims any right over the property which is being sold then you will indemnity and protect him against all such claims at your costs and expenses
4. Before you proceed with the above course of action, you may have to get your name entered in society records like share certificate issued for your late mother's flat. So get that done from the society first.
5. But society may also insist for Probate of your mother's Will in which case you can suggest it the same course of action as given in para 3 above.