By Rabbi Berach Steinfeld
In Parshas Lech Lecha we find that after Avrohom conquered the kings in war he met Shem ben Noach who was the Kohen of the generation and it says that Avrohom gave Shem maaser from everything he received.
In Parshas Toldos it says that after leaving Avimelech, Yitzchok planted a field and was so successful with its planting that it produced “Meah Sharim” 100 times more. Rashi explains why Yitzchok measured the field’s production. The reason is so he could give maaser on what grew.
In Parshas Vayeitzei, Yaakov promised Hashem that everything you will give to me “Aser Aasrenu Lach.”
The Baalei Tosfos bring down the Medrash that says that Yaakov established the concept of maaser from money earnings. That is where we learn the concept of giving a fifth of one’s money to tzedaka. We learn this from the double lashon of aser aasrenu; two times a tenth, which is a fifth. The Rambam in Hilchos Melachim, Perek Tes, halacha alef says that Yitzchok established maas
There is a machlokes Rishonim regarding giving maaser from monetary earnings whether the din of giving maaser from money is the same as giving maaser from grain. Is giving money considered tzedaka with a base minimum of one tenth and a maximum of a fifth of one’s money?
We could try to reconcile this by saying that each of our Avos established a different part of maaser. Avrohom gave maaser
The Shita Mekubetzes in Kesubos, daf nun, amud alef says that when one earns money he should take off a tenth and use it for his own religious purposes. He should then take off another tenth and use it to give to the poor. He explains that it is similar to the two maasros we give on grain or fruit. One maaser was taken up to Yerushalayim for personal consumption and the other maaser was given to the poor. It could be that this is what Yaakov meant with the double lashon of “aser aasrenu lach.” He did not say “Achamshenu;” I will give a fifth. He said, “I will give two tenths.” We know that one may not take maaser from a crop that was already tithed in order to fulfill maaser for another crop that was not yet tithed. Hence we learn that the two crops cannot be mixed together until both are properly tithes. Similarly, one may not be mafrish a fifth for two tenths he wants to give maser on, he has to do it in two steps; first one tenth and then another tenth.
We see the great value in giving maaser when the Gemara explains “Aser bishvil Shetisasher.” One who gives maaser will become rich. Maaser is the perfect investment.