Q. If I know I will be getting a wages cheque at the end of the month or any other future income, can I give the maaser money already now to the poor and have it count for future moneys not yet earned?
A. Noda Beyehuda (Y.D. 1: 73) quoted by Pischei Teshuva (T.D. 249: 1)mentions that once it became an established custom, the giving of the tzedaka to the poor can be counted as maaser. This is also the opinion of the Chasam Sofer in his chiddushim to Maseches Gittin 31b. Derech Emuna (P. 76) writes that this was also the custom of the Chazon Ish
It would seem that Avraham Avinu's giving maaser to Malki Tzedek (P. Lech Lecha), even when he didn't accept the money offered by the King of S'dom meets this description.
However, Horav Shlomo Miller's Shlit'a opinion is that Avraham Avinu accepted the moneys that were given to others and for that he gave maaser.
The Rov pointed to Shaarei Aharon who quotes that Avraham Avinu gave maaser again from his own wealth, since he could have lost all in the war, but miraculously all was saved. In his eyes it was like gaining it all again.
He also mentions that since the King of Sdom would have lost all, when the Patriarch saved it, automatically it became his (matzil min hagayes). Although he gave it all back, since he technically earned it, he gave maaser from it.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit'a
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