Rectification of illegal and invalid transfer of membership

The housing society transferred the flat in the name of purchaser on the basis of un-stamped and unregistered conveyance deed dated June 1989. Thereafter in Dec 2018 the purchaser came to know by taking some legal opinion that the transfer made by the society is invalid and illegal under section 53 A of Transfer of Property Act since the conveyance deed submitted was unstamped and unregistered. The Society has no jurisdiction to entertain any such invalid / illegal document, which do not have any value in law. Unregistered Original Conveyance Deed dated June 1989 therefor does not transfer a valid title of ownership to the purchaser / transferee.

Please advise, how can we now approach the society to cancel the wrong transfer/endorsement made in the share certificate and re-transfer the flat in the name of Original Member of the society. Thanks