This-worldly reward as Divine gift

ובמד' ג' דברים במתנה התורה והגשמים כו'.

And in the Midrash: three things were given as a gift (matanah) - the Torah, and the rains, etc.

The Sefas Emes cites the Midrash that lists several things - among them the Torah and rain - that Hashem grants as outright gifts rather than as earned wages.

פי' התורה א"י להשיג ע"פ זכות האדם רק במתנה וגשמים פי' זה כי שכר מצוה בהאי עלמא ליכא רק רשעים משלמים להם בעוה"ז אבל הצדיק ע"פ דין בעל חסרון הוא בעוה"ז רק התורה מבטיחה כי הקב"ה יתן במתנה לכל החוסים בו כי הוא כל יכול ולכן הבטחת עוה"ז לצדיקים היא מתנה גמורה.

That is: the Torah cannot be attained by a person's merit, only as a gift; and "rains" - the explanation is this: for "there is no reward for a mitzvah in this world" (Kiddushin 39b); only the wicked are paid in this world, while the tzaddik, by strict law, is "lacking" in this world. But the Torah promises that Hakadosh Baruch Hu will give as a gift to all who take refuge in Him, for He is all-capable; therefore the promise of this-worldly [reward] to the tzaddikim is a complete gift.

The Torah cannot be earned by merit - it comes only as a gift. Similarly, since there is no payment for mitzvos in this world (the wicked are paid here precisely to deprive them of the World to Come, while the righteous are deliberately left wanting), the material blessings the Torah promises the righteous in this world cannot be "wages." They are a pure, additional gift that Hashem, who is all-powerful, bestows on those who shelter in Him.

ובנ"י זכו לזה ע"י קבלת התוכחה.

And Bnei Yisrael merited this through accepting the rebuke (tochachah).

Bnei Yisrael earned this gift of this-worldly blessing specifically by their willingness to accept the harsh admonition of the tochachah.

כי גם עונשים אינם בעוה"ז ע"פ דין.

For punishments, too, are not [meted out] in this world according to strict law.

Just as reward is not given in this world by strict justice, neither is punishment - by the letter of the law, neither belongs to this world.

רק שבנ"י מקבלים באהבה עונשין כדי שלא יתרחקו ח"ו מדביקותם בהקב"ה ע"י החטאים.

Rather, Bnei Yisrael accept punishments with love, so that they should not, Heaven forbid, become distanced from their attachment (deveikus) to Hakadosh Baruch Hu through their sins.

Bnei Yisrael lovingly welcome this-worldly suffering precisely because it cleanses their sins and keeps them from being pushed away from their deveikus with Hashem.

ובקבלה זו זכו למתנת עוה"ז ג"כ בעשותם הטוב.

And through this acceptance they merited the gift of this world as well, when they do good.

Because they accept punishment with love in order to preserve their bond with Hashem, they correspondingly merit that Hashem grants them this-worldly blessing as a gift when they do good.

והוא עצה מהש"י לבנ"י כמ"ש נפלאותיך ומחשבותיך אלינו:

And this is a counsel (eitzah) from Hashem to Bnei Yisrael, as it says, "Your wonders and Your thoughts are toward us" (Tehillim 40:6).

This whole arrangement - giving reward in this world as a gift and using punishment as a loving means of preserving connection - is Hashem's loving plan on behalf of Bnei Yisrael, expressing that His every "thought" is directed toward their ultimate good.

Summary: Because there is no earned reward for mitzvos in this world - the wicked are paid here while the righteous are deliberately left wanting - the material blessings the Torah promises tzaddikim cannot be wages but are a pure gift, like the Torah and rain themselves, given to all who take refuge in Hashem. Bnei Yisrael merited this gift by accepting the tochachah's punishments with love, embracing this-worldly suffering as a means to cleanse sin and safeguard their deveikus with Hashem. The entire arrangement is Hashem's loving counsel for His people, whose every Divine "thought" is directed toward them.