It is advisable to seek the help of an advocate by providing all the documents. The General Power of Attorney is registered and the seller has acknowledged the receipt of full consideration through a notarised affidavit. Power of attorney coupled with consideration cannot be revoked. You may reply the legal notice through your advocate and also issue a paper notification to avoid any unfair play. But in the absence of a sale deed, in favour of the allottee, by BDA, and as the allottee was only lessee when the site was purchased by your friend the matter is complicated. It is advisable to sort out the issue with the allottee. Alternatively you may also claim the amounts paid with upto date interest.