Purchasing an apartment flat

Dear Sir/ Madam,

I am a bangalorean and purchasing flat in an apartment. The land belongs to an NRI and the builder is constrcting first apartment. Builder has approved CC and all documents looks fine and verified by a lawyer. There are about 43 flats belongs to Builder and remaining 21 out of 64 belong Owner share. 

My question:

The flat i have liked is belong to Owner share. However builder is quoting that, Owner is planning to sell all the flats to Builder itself as he is residing outside India. This will hapen in 2-3 moths time. 

Now since I have liked the flat, builder want to give it to me if I confirm whether I really want to purchase[by giving token advance]. he is planning to swap the one i liked with other which belong to builder. As a documentation, he is going to get a bond paper fully signed by land owner. With this leagally, he is swapping the flat[ of same size ]. I ultimately get the flat from Builder. 

What other document do i need to get from owner?. Builder has GPO. Does the bond paper mentioning swapping the flats is sufficient?. Anything else i need to consider.