Parashas Masei · Passuk 36:6 · TT note 1
Chabad.org — English translation + Rashi
Torah Temimah points to Ta'anis 30b (TT note on “זֶה הַדָּבָר”).
English: It was taught: Rabbi Shimon ben Gamliel said, on the fifteenth of Av the tribes were permitted to intermarry with one another. What did they expound? “This is the matter” — this matter shall apply only in this generation.
Time-bound restriction on inter-tribal marriage limited to the land-division generation.
The requirement that heiresses marry within the tribe was for the generation of the conquest; from 15 Av onward inter-tribal marriage was permitted.