1. Does the MOU legally remain valid after a successful execution of the assignment from the seller to buyer with all payments made and deal closed? (to our understanding it holds good only till the assignment agreement is executed)
Once the property is transferred by an assignment deed the original buyer's responsibility or liability over the property terminates and to evidence this is the present MOU or any tripartite agreement is made.
Therefore, since all the issues have been sorted out and agreed at the time entering into a MOU, the demand now being made with you is not maintainable.
2. Does the builder have a legal right to come back after an year of successful execution of assignment agreement and ask for that sum of money?
Dont entertain any verbal or oral demand. Let him send a legal demand notice, you can give a fitting reply based on the agreement already signed in this regard.
Let him seek the relief through court and let the court decide based on the facts and merits.
3. Can a successful assignment agreement be executed by the builder between buyer and seller if all the payments were not closed by the seller & agreed upon to be buyer to pay the same before assignment?
It is not abnormal. It is usual practice being followed by many under such circumstances.
There is no legal infirmity in it.
4. Do I have any legal option to deny that payment since the property is no longer in my name. Per my understanding, if payments were due from my side, the deal and execution of assignment agreement wouldn't have successfully taken place. Once the property has been transferred to the buyer, it should be a matter between builder and the buyer. Seeking your expert legal advice on the same.
Your understanding is right. You can very well deny or refuse to pay any amount, let them make it legal issue, they may not be successful to recover the same from you.