וזהו שאמרו רז״ל: אין ישראל נגאלין אלא בצדקה
And this is what our Sages, of blessed memory, meant by saying that7 “Israel will be redeemed only through charity.”
שיעשו גם אם יהיו פטורים מדינא
[This refers to the charity] that they will perform even if they are legally not obligated,
כי אין בן דוד בא כו׳
for8 “[Mashiach] the son of David will not come [until the pocket will be empty of even the smallest coin].”
I.e., even if (Heaven forfend) there will not be a solitary coin left in one’s pocket, tzedakah will still be given. And it is this boundless level of tzedakah that secures a complete atonement for the sins of our people, after which9 “they will immediately be redeemed.”
The Rebbe explains that the Alter Rebbe does not conclude the above-mentioned quotation about the precondition for the coming of Mashiach because it is quite possible that he did not want to write out the last words (viz., “until the pocket will be empty of even the smallest coin”); and this precondition of the Sages can be fulfilled on the spiritual level, by conducting oneself with the humility of the destitute.
This could also explain why the Alter Rebbe does not say...כשיהיו (“when they are legally not obligated”), but rather...אם יהיו (“if they are legally not obligated”).
7. See above, Epistle 9, footnote 16.
8. Sanhedrin 97a.
9. Rambam, Hilchot Teshuvah 7:5.
From: Lessons in Tanya