1)
What is "Nosen la'Rash"?
1.
Rashi #1: He gives Tzedakah.
2.
Rashi #2: It is a Rebbi who do not withhold Torah from his Talmid.
2)
How is there no lack?
1.
Orchos Tzadikim (Sha'ar ha'Nedivus DH v'Da): One who has this Midah, he becomes rich.
2.
Malbim: Hashem will bless him due to this (so the giver will not lack). Also, he will not lack from his wealth, for 'salt (the preservative) of money is Chaser' (Kesuvos 66b).
3)
What do we learn from "u'Ma'alim Einav Rav Me'eros"?
1.
Malbim: One who hides his eyes from [seeing the need to give] Tzedakah, not only will he not be blessed - many curses will be sent on him.
