Maaser sanctifies the whole through its part

עשר אעשרנו לך.

"I shall surely tithe it to You" (Bereishis 28:22).

As part of his vow, Yaakov pledges to set aside a tenth of all that Hashem grants him; the Sefas Emes uncovers the deeper meaning of maaser (tithing).

פי' שיהי' הכל מקושר בכח המעשר.

The meaning is that everything should be bound together through the power of the maaser.

By designating a tenth for Hashem, the entire remaining portion becomes connected to and elevated through that holy tenth.

כי הקב"ה נתן מצות המעשר כדי להיות חלק לקדושה בכל הדברים ועי"ז נמשך קדושה לכל המשויר.

For the Holy One, blessed be He, gave the mitzvah of maaser so that there should be a portion of kedushah within all things, and through this kedushah is drawn into all that remains.

The purpose of maaser is to place a "share of holiness" into one's possessions; that consecrated portion then channels kedushah into the rest, so the entire whole is sanctified through its one holy part.

והצדיק שמבטל הכל אל המעשר והחלק גבוה חשוב אצלו יותר מהשאר לכן נכלל הכל בהמעשר וז"ש עשר אעשרנו וכ"כ עשר תעשר והבן:

And the tzaddik, who nullifies everything toward the maaser — and the portion belonging to On High is more precious to him than the rest — therefore the whole is included within the maaser. This is the meaning of "I shall surely tithe" (asser a'asrenu), and likewise "you shall surely tithe" (asser te'asser, Devarim 14:22); understand this.

The tzaddik practices bittul, treating the "portion of the Most High" as more valuable than everything he keeps for himself; because his heart is bound to that holy tenth, all his possessions become subsumed within its kedushah. This explains the doubled language of the Torah's verbs for tithing — asser a'asrenu and asser te'asser — which hint that through the maaser the entire whole is elevated.

Summary: Yaakov's pledge "I shall surely tithe" reveals the inner purpose of maaser: by consecrating one tenth to Hashem, a "portion of holiness" is placed within one's possessions that draws kedushah into all the rest. The tzaddik, through bittul, cherishes that holy portion above everything else, so that all he owns becomes bound up in and elevated by the maaser — the meaning hinted at by the Torah's doubled language of tithing.