How can a man bring a gift to Hashem, if everything belongs to Hashem?
He brings his heart as a gift to Hashem, that's what you're bringing. When you bring bikurim, you're bringing your heart to Hashem, when you're bringing a korban, you're bringing yourself to Hashem. It's like saying, Ribono shel olam I'd like to offer myself as a korbon to the miazbeiach, but You won't take it. You told Avrohom, al tishlach yodcho el ha'naar, v'aal ta'as lo m'umo, don't do anything to Yitzchok, so you don't want any offerings. So ma ashiv Lashem, how can I pay back Hashem? Ha'eten bechori pishi (Micha 6:7) can I give my firstborn as an atonement for my sins? No, I can't do that, so I'll give a bechor of my korbonos.
So we see that a korbon is giving yourself to Hashem. So if you give maaser, it's really giving yourself to Hashem. It's the heart that Hakadosh Baruch Hu wants of you, and your heart you can give.
Heart means your emotions, your love, your devotion to Hashem. That's what you give when you give anything to Hashem.
This astonishing statement defies all logic. Hashem is infinite and not physical, and His Reality transcends any space-measurement. This therefore actually means that Israel is the purpose of the Universe, and Hashem here declares that this group of men is the center of His thoughts. This holds true for all future generations of Israel: "For the mountains may depart and the hills be removed; but My kindness shall not depart from you, nor shall My covenant of peace be removed, says Hashem that loves you" (Isaiah 54:10). But in the intensity of Hashem's Presence, no generation ever gained a similar recognition by Hashem.
The statement "And I shall reside in their midst" in the sense that it was made here, was never approximated afterwards. This was the generation of generations. It must be emphasized that never again subsequently was Hashem's presence as openly evident as it was at this time.