The mitzvah of bikurim entails “v’smachta b’kol hatov” – bikurim are called “tov.” The gemara (Archin 11a) even has a hava amina that the pasuk that refers to avodah “b’simcha u’vtuv leivav,” from where we learn shiras haLevi’im, is perhaps instead referring to bringing bikurim, since they are also called tov.
What Moshe was promising Yisro, writes the Rogatchover, was the opportunity to bring bikurim. The Yerushalmi (Bikurim 1:4, 3a in the Vilna ed) writes:
ובני קיני חותן משה מביאין וקורין דכתיב (במדבר י) לכה אתנו והטבנו לך.
When Rashi in Beha’alosecha writes that Yisro was given a portion of land near Yiricho for safekeeping, it doesn’t mean the land itself is the tov that Moshe promised. In light of the Y-lmi what Rashi means is that since Yisro’s descendants were able to meet the necessary precondition of owning land, therefore they could fulfill the mitzvah of bikuim, "v'samachta b’kol hatov."