Sale of Unregistered Flat

I booked an apartment in Bangalore which was worth 48Lac Rs. in Dec 2016 by paying an Initial amount of 12Lac Rs. Later paid in installments upto 45 Lac Rs. in total, the last installment was paid in Feb 2019. There is a remaining amount of 3Lac Rs. to be paid to the Builder as per the initial agreement and get the Possession. Later which register the property in my name. Apartment is in Ready to Occupy state. Builder is requesting me to pay the remaining amount or sell to a new Buyer. I'm in a financial situation where i cannot pay the remaining 3Lacs and get the property registered in my name. I have identified a Buyer and he is ready to buy the apartment for 64 Lac and register in his name. In this case, the builder will execute the Assignment deed and transfer the property from my name to the new Buyer's name. Am i eligible for Capital Gain Benefit here?. If yes, i shall park the Gain Amount into a Capital Gain Account and use the remaining amount. If No, Then the Gain i had made will be shown as "Income from Other Sources" and pay Tax as per my tax slab ?.Am i right here? Correct me if wrong. What are the ways to legally save tax in this case.