(19) All the sacred gifts that the Israelites set aside for Adonai I give to you, to your sons, and to the daughters that are with you, as a due for all time. It shall be an everlasting covenant of salt before Adonai for you and for your offspring as well.
Rashi - Rabbi Solomon ben Isaac, France, 1040-1105
It shall be an everlasting covenant of salt — This means that God made a covenant with Aaron by means of something which is wholesome and lasting and keeps other things wholesome.
Rashbam - Rabbi Samuel ben Meir, France, 1085-1158
I believe that the Torah uses the term “salt” to symbolize something which endures indefinitely... a covenant meant to
endure permanently, throughout the ages.
Rav said: Any meal in which there is no salt is not considered a meal.
Soferim 15:8
As the world could not do without salt, neither could the Jewish People do without the Torah
Numbers 18:26-32 (selections)
(26) Speak to the Levites and say to them: When you receive from the Israelites their tithes, which I have assigned to you as your share, you shall set aside from them one-tenth of the tithe as a gift to Adonai. (27) This shall be accounted to you as your gift...You shall set aside all gifts due to Adonai from everything that is donated to you, from each thing its best portion, the part thereof that is to be consecrated. [Of the main part] you and your households may eat it anywhere, for it is your recompense for your services in the Tent of Meeting. (32) You will incur no guilt through it, once you have removed the best part from it; but you must not profane the sacred donations of the Israelites, lest you die.
Rabbi Even Posen - Congregation Neveh Shalom, Portland, Oregon
The lesson is that even those who rely on public support for their livelihood must give part of what they receive as tzedakah. The act of giving is one that can nourish the soul of the giver as well as sustain the receiver.
Maimonides - Moses ben Maimon, Spain/Egypt, 1135-1204
Everything that is for the sake of G‑d should be of the best and most beautiful. When one builds a house of prayer, it should be more beautiful than his own dwelling. When one feeds the hungry, he should feed him of the best and sweetest of his table. When one clothes the naked, he should clothe him with the finest of his clothes. Whenever one designates something for a holy purpose, he should sanctify the finest of his possessions, as it is written, “Each thing its best portion.”
Rabbi Moshe Bogomilsky, Vedibarta Bam
Some people, unfortunately, do not give tzedakah because they are reluctant to give away part of their wealth. The Torah, therefore, is telling us that tzedakah is like salt: it causes meat to shrink somewhat, but the preservative effect far outweighs the loss. Similarly, though on the surface it appears that one’s net value is shrinking, but in reality, thanks to the tzedakah, one’s earnings are preserved.
Ketubot 66b
Rabbi Yochanan ben Zakkai was once traveling on the outskirts of Jerusalem. A young girl who was picking barley from the garbage in the street approached him and said, “Rabbi, please support me.” He asked her, “Whose daughter are you?” She replied, “I am the daughter of Nakdimon ben Gurion.” Rabbi Yochanan ben Zakkai knew him by reputation to be one of the wealthiest people in Jerusalem, so he incredulously asked her “What happened to all your father’s wealth?” She responded with the famous parable: “melach mamon, chaseir!” [This means] if a person wants to “salt” his money (i.e. preserve it to be long-lasting), he should reduce his assets by giving some to tzedakah. Unfortunately, her father did not properly observe this mitzvah, and therefore his fortunes were not preserved.