1. Your flat does not exist in any documents so far made available with you.
2. Unregistered agreements/deeds in connection with real estate have no value legally.
3. Get the development agreement registered incorporating the inclusion of your flat no. and get the share of the vendor and developer specified there itself.
4. If the development agreement was registered earlier, register a supplementary development agreement incorporating the inclusion of your flat no. and get the share of the vendor and developer specified there itself.
5. Once your said flat no. start featuring in the said property documents you should proceed to buy the same.
6. Avoid taking all shot cut steps voiding the legally acceptable steps in this regard.
7. Apply for bank loan for buying the aid property in which case, the Bank lawyer will conduct search and certify whether it is legally safe to provide loan to you or buying the said property or not.