Real Estate Transaction - Income Tax and GST

A Builder entered into Sale Agreement with a Customer 'A' in 2016 for Sale of a Flat for Rs. 60 Lakhs. There is a clause in the Agreement that in case of financial difficulty faced by the Customer (A), either the Builder can buy back the Flat or Customer A can transfer the Flat to another person by executing MOU/Assignment Agreement. Customer A had availed Loan from a BANK for the purpose of this Flat.

Now Customer A finds another Person 'X' to whom he transfers the flat by executing MOU for Rs. 80 Lakhs. Bank also agreed to fund to the new person X.

Now the Builder shall mention all these matters in FINAL SALE DEED and shall receive balance due from original customer (A), along with applicable TDS & register the flat to new customer name (X) for 80 Lakhs. The Builder will receive balance from original customer (A) & customer A will receive balance sale consideration from his customer X.

1..WHAT WILL BE THE IMPLICATION OF INCOME TAX AND GST IN THIS CASE? 

2..IS THERE ANY BETTER METHOD TO CARRY OUT THIS TRANSACTION?