Parashas Ki Savo · Passuk 26:1
And it shall be when you come to the land that Hashem your God gives you as an inheritance, and you inherit it and dwell in it.
Chabad.org — English translation + Rashi
Torah Temimah points to Sifrei Devarim 297.
English: “And it shall be when you come to the land” — the expression “and it shall be” indicates immediacy. “When you come to the land” — perform the mitzvah stated in this matter, for in its merit you will enter the Land. “Which Hashem your God gives you as an inheritance” — R’ Shimon says: this excludes Transjordan, which they took for themselves. “And you inherit it and dwell in it” — once you inherit, you shall dwell.
R’ Eliezer #31 (סמוכין) — juxtaposition of the coming into the Land with the mitzvah that follows; the entry is presented as the reward for performing bikkurim. Secondary use of limiting language (פרט) for Transjordan.
The mitzvah of first fruits is the merit through which Israel enters its inheritance. The obligation (and the corresponding promise) applies only after inheritance and dwelling, and only west of the Jordan.