If congress sends to the president a bill with both houses having approved of the bill with 2/3 margin?

It's all in Article I, Section 7 of the constitution. If the President wants to veto the Bill, he must do so within 10 days, or it becomes law anyway. If he vetoes it and both Houses pass it by a 2/3 majority, it becomes law.

It doesn't mention what happens if they pass it by a 2/3 majority originally, but the strict wording would suggest that they have to pass it again if the President vetoes it.

1) If a president signs a bill it is returned to the house which sent it and becomes law. That seems pretty obvious!

If the president vetoes a bill, i.e, does not sign it, he returns it to Congress unsigned. Only if both houses of Congress subsequently vote by two-thirds majorities to override the veto does the bill becomes law. If one (or both) houses do not vote to override the veto it does not become law and the veto is upheld.

2) The president is required to return a bill to Congress within 10 days excluding Sundays. If he does nothing, i.e., does not return the bill to Congress signed or unsigned, it becomes law.

3) If the president can not return the bill to Congress because it is on recess, the president has 'pocket vetoed' the bill. From wikipedia:

"A pocket veto occurs when a bill fails to become law because the President does not sign the bill and cannot return the bill to Congress within a 10-day period because Congress is not in session. Article 1, Section 7 of the U.S. Constitution states:

'If any Bill shall not be returned by the President within ten days (Sundays excepted) after it shall have been presented to him, the same shall be a Law, in like manner as if he had signed it, unless the Congress by their Adjournment prevent its return, in which case it shall not be a Law.'"

4) Members of Congress are acutely aware of this tactic as it gives the president the functional ability to 'pocket' the bill. To prevent a pocket veto "Congress can adjourn and designate an agent to receive veto messages and other communications so that a pocket veto cannot happen, an action Congresses have routinely taken for decades".

5) The answer to your question is 'no, it does not become law automatically'. If the president signs the bill, it is returned to Congress and becomes law.

Otherwise:

If the president does nothing it becomes law in 10 days.

If the president returns the bill unsigned (veto) both houses of Congress must vote to override the veto by a 2/3rds majority for the bill to become law.

If the president attempts a pocket veto, an agent of Congress will (or should!) prevent it from working. The result is a normal veto which must be overridden as above to become law.

Louis is WRONG.

The bill is not law until the president signs it, OR unless Congress overrides a veto, regardless of how many votes it got the first time around.

And since the president is constitutionally required to EXECUTE the laws that Congress passes (hence the term EXECUTIVE BRANCH OF GOVERNMENT) he has to READ all of them.

The bill is to approve the agreement and lift the sanctions. Someone at the White House reads the whole bill.

Techniy it still has to be signed by him but he is required to sign it so it's essentially a done deal once a veto proof majority signs it

He would be wasting his time if he vetoed it. Still, he has to sign it into law.

Oh, I guess he could renege, but it would still become the Law. It might delay it for a few days but the President would just be demonstrating he is a fool so it is unlikely he would not sign it.

It needs his signature. If he vetoes it, another vote would need to be taken. It's quite possible that the percentages could change before the vote.