מדרש אויף ויקרא 2:14

(Exodus 22:24) "Im you lend money to My people": R. Yishmael says: Every "im" (generally "if") in the Torah connotes optionality, with the exception of this and two others; (Leviticus 2:14) "And im ("when") you offer a meal-offering of first-fruits," where it is mandatory. You say it is mandatory, but perhaps it is optional? It is, therefore, written (Ibid. 14) "You shall bring the meal-offering of your first-fruits." It is mandatory and not optional.

1) (Vayikra 2:14): "And if you offer a meal-offering of first fruits (bikkurim) to the L–rd" (the meal-offering of the omer): R. Yehudah says: The meal-offering of bikkurim is destined to be suspended (with exile) and to be restored. And, similarly, it is written (Bamidbar 36:4): "And if the jubilee (yovel) will be for the children of Israel" — the jubilee is destined to be suspended and to be restored.

1) (Vayikra 2:14): ("Groats of the fresh ear [karmel]) you shall offer (the meal-offering of your bikkurim"): Why repeat this? (It is already mentioned in the beginning of the verse.) Because it is written "karmel," I might think the mitzvah is to bring only rach mal (see Vayikra 2:8) above). Whence do I derive that if he did not find rach mal he may bring it dry? From "you shall offer the meal-offering of your bikkurim" (in any event). It is a mitzvah to bring it from the harvest (i.e., from the standing grain). Whence do I derive that if he did not find this he may bring it from the (sheaves) in the loft? From "you shall offer the meal-offering of your bikkurim" — in any event.