From Spirit of Matter (#16) by Rabbi Avigdor Miller
Hashem gives us this world happiness. He gives us peace, He gives us good health. He’s not paying us off. But He’s encouraging us. It’s like a man who has a machine, a valuable machine that’s turning out precious products. He’s standing over his machine with an oil dropper, and he drops drops of oil to lubricate the machine so it should work more efficiently. “schar mitzvah mitzvah” (Avos), the payment for a mitzvah is opportunity to do more mitzvos.
When you give charity, when you give tzedaka, Hashem promises He’ll give you more money. That’s not reward, because if you get more money you’ll have an incentive to give more charity. The gemara says “rodef tzedaka v’chesed yimtza chaim tzedaka v’kavod — If a man pursues tzedaka, charity, and kindliness to other people, he’s going to find life, tzedaka and honor." What does that mean?
The payment for a mitzvah is opportunity to do more mitzvos
If a man likes to give money for good, Torah causes, he likes to give to yeshivos, Hashem is going to encourage him. First of all, He’ll give him life. Instead of seventy years of giving charity, He’ll give him eighty years, ninety years: that’s encouragement. It’s like a worker who works overtime for his boss. He produces more. So that generous person will get more and more opportunities to give tzedaka. Hashem sends him more money and more opportunities, and he’ll give more and more.
And kavod: he’ll get more honor. Since he’s a charitable man, Hashem gives him influence, and he goes around asking people to donate. Everybody respects him, and they give.
But when a man is in a hurry to get paid in this world, that’s a different story. It’s a misfortune because this world doesn’t have enough money to cash in the check from the bank of Olam Haba.