Thank you, again.

I spent all day reading about this again.  In Greek.  The Greek version of the Treaty definitely states Excluded.  There have been numerous Court decisions, etc. over the years, but I am none the wiser about this after my research.  There was an agreement some years ago that a dual citizen living in the U.S. can take certain steps to have the Greek Government not tax the Social Security at all and remove from the Tax Form (the form always has the social security amount received and the taxes withheld pre-entered on the Tax Form of each individual).   However,  that is very complicated involving residence documents and the exact amounts received the previous year, which after signature by the IRS need to be apostilled and submitted personally in Athens up to the end of February every year.   This has to be done annually, which is very difficult if you live in the U.S. 

For the first client, the dilemma was 2,000 difference so I wanted to be sure.  It did not seem right or fair for such a difference in taxes, but then the taxation law is not always fair.

I am very grateful to you for taking the time and effort to give me the answers.   Especially for the explanations.