I don't know if Leo GREENBERG is a relative. All I have is my grandfather's death cert. in 1912 in NY, which says his mother was Rachel GREENBERG. It's possible Leo was related to her, but I have no way to know that so far.
Do you know anything about his family history in Wysoke-Lit.? I think Rachel must have been born in the 1840s. She married Ovsie (Shia) DAJBOG by 1859 or so. (I haven't been able to find any trace of him either except his name as my grandfather's father). My grandfather was Schmuel Ber DAIBOCH in W-L and became Sam FEINBERG in NY.