Earlier in the Torah, Rashi mentions three instances in the Chumash where the word "םא," literally "if," is employed not as a condition but as an imperativeבד"ה ואם( ב"כ:'כ תומש). One of the three is in this week's Parsha, which deals with the offering of the Korban Omer. The Torah says "...םירבכו תחנמ בירקת םאו," which is understood to mean, "when you bring a meal-offering of the first grain..." (ויקרא ב':י"ד). The wording of this Posuk, which makes it sound like this offering is optional when it in fact is obligatory, can be explained as teaching that a person must ultimately bring a sacrifice willingly, and not merely because it is obligatory. Consequently, it is as if one does have a choice in the matter, and, as such, the Posuk is worded conditionally. Likewise, one should not consider davening as a burden, which must be recited only because he is obligated, but one must rather daven out of love for Hashem.
The same idea applies to lending money and to other matters which are ורבחל םדא ןיב, between man and his fellow, and which must consequently be performed willingly and cheerfully. The Bartenura, in his commentary on the Mishnah in Pirkei Avos (פרק א' משנה ט"ו) states that if one gives Tzedakah with "his face pressed to the ground," that is, under duress, it is as though he did not give at all. Although he will certainly receive a reward, since he gave because of Hashem's Mitzvah and feels compelled, he has not performed the Mitzvah in the manner which Hashem wishes, namely, willingly and lovingly.
Rav Moshe Feinstein writes in his Sefer Darash Moshe that the same idea applies to the Mitzvos of Bikkurim, bringing the first fruits to the Kohein, and the Korban Omer, the Korban from the first of the new year's harvest. The owner, who worked hard to see his fruits grow, surely enjoys them more than anyone else, and they mean much to him. Why, then, does the Torah command that they be given to the Kohanim? The answer is that the fact that the owner gives away the fruits of his labor and toil and that which he looked forward to harvesting is the greatest sign that he is performing these Mitzvos willingly and lovingly.
It was for this very reason, Rav Feinstein continues, that Kayin was punished when he brought his inferior crop as a sacrifice (בראשית ד':ה'). He felt that the quantity or quality of what one brings as an offering makes no difference to Hashem. Accordingly, he reasoned that bringing inferior produce was acceptable. It is only to man, he thought, that quality makes a difference. Kayin was punished because he misunderstood the essence of sacrifices. It is, of course, a fact that Hashem does not need anything, because everything is His, but the whole idea of a sacrifice is that one should feel that he is giving away what is dear to him because that is Hashem's will, and as such, he must offer his best.
Therefore, one must dedicate the Bikkurim, the first and the best, to Hashem, for although to Hashem they mean very little, to the person who is offering them they mean a great deal. For that reason, it says "םא" in the context of Bikkurim and the Korban Omer to teach that one should give his best with the attitude that ultimately, one has free choice whether or not to give, and that despite there being an obligation, he still gives of his own free will. The eagerness with which he fulfills the Mitzvah attests to his righteousness and his faith.