What is the significance of the fact that Hashem only instructed Moshe to sacrifice the Nesi'im's Korbanos after the Nesi'im brought them in front of the Mizbe'ach?
Ramban (13): Here we have a case where Hashem acceded to the request of the Nesi'im and turned their good intentions into a Mitzvah 1 for all times, to inaugurate the Mikdash and the Mizbe'ach; for, in similar vein, Shlomo ha'Melech made a Chanukas ha'Bayis when he built the first Beis-Hamikdash, 2 as did the Anshei Kenesses ha'Gedolah when they built when they built the second. 3 And in the time of Mashi'ach too, there will be a Chanukas ha'Bayis for the third Beis-Hamikdash, as the Pasuk states in Yechezkel, 43:26, 27.
QUESTIONS ON RASHI
Rashi writes that Moshe did not know if the Nesi'im will offer according to seniority (the oldest Shevet first), or according to the order in which they encamped. Why was he unsure? Hashem commanded about the Degalim only a month later! (Bamidbar 1:1 was on Iyar 1, and Perek 2 teaches about the Degalim.)
Mizrachi: Hashem already told Moshe, during the last 40 days on Har Sinai, about taking Levi in place of the Bechoros, and the Degalim.
Gur Aryeh: There is no need to say so. The Shevatim already had a tradition from Yakov about how they will camp, like Rashi wrote above (2:2)! (Bamidbar Rabah (2:8) says that Moshe expected the Shevatim to complain about the directions in which he will tell them to camp. This implies that Moshe did not have this tradition before the command about the Degalim! - PF)