My grandfather attributed this to his grandfather's grandfather.
According to Anne Kandel, this was Itsyk Finkelstein, born in Kedainiai, Lithuania in 1785.
The last two sentences of the first paragraph of the מגיד section of the Haggadah read
השתא הכא לשנה הבאה בארעא דישראל
השתא עבדי לשנה הבאה בני חורין
This year we are here; next year may we be in the land of Israel.
This year we are slaves; next year may we be free.
Both sentences mean the same thing; they are a prayer for the coming of the Messiah. So why do we say it twice?
The answer is that there are two schools of thought; one school believes that the Messiah will come in Nisan, at Passover; the other believes he will come in Tishrei, at Rosh Hashana. Both schools agree that, six months before the coming of the Messiah, Elijah the Prophet will come, and liberate the Jews from their oppressors; when the Messiah comes, six months later, he will gather the Jews to the land of Israel.
It is all too obvious that neither Messiah nor Elijah has come yet. According to the first school, we may hope that the Messiah will come by next Nisan, and Elijah will come this coming Tishrei. Hence, "This year we are here; next year we will be in the land of Israel". According to the second school, the best we can hope for is that next Nisan, Elijah will come, and that the Messiah will come in Tishrei, a year and a half from now. Hence, "This year we are slaves; next year we will be free."
The debate over whether the Messiah will come in Nisan or in Tishrei is in Rosh haShanah 11a. I am grateful to Yehuda Herskowitz for the reference. I do not know any source for the theory that Elijah will come six months before the Messiah.