Dear Mr.Arvind,
The sale deed must be unconditional. The Vendor cannot impose such conditions which takes away the absolute right of the purchaser, since the sale is against a valuable consideration. Its the onus of the purchaser to go through the draft sale deed before signing the original deed. generally, the purchasers do not read the draft sale deed and blindly sign the registered document out of trust.
Its always advisable to vet the draft before signing the registered deed.
In your case, this clause which talks about getting an NOC from the society is void since it is illegal. But since your mother had signed the registered deed, she has agreed to this clause and hence the best remedy is to seek legal relief from the court.