In the beginning the Avodah of Karbonos was with the bechor (see the mishnah in the last perek of Zevachim. The avodah was done by the bechorim until the Mishkan was built) until after they made the egel ha’zahav. Then the Avodah was given to the Kohanim and Shevet Levi. They (Kohanim) received matnas kahunah (chozeh, shuk, reishes hagez) because of their service in the Mishkan (and Beis Hamikdash.)
Before the egel ha’zahav to whom would these presents be given to? The bechorim?
Answer:
We cannot answer this question with certainty, because the sacrificial service of the Mishkan was only initiated after the sin of the Golden Calf and after the Kohanim were elected as those who will perform the service of the Mishkan and later the Mikdash.
However, the pasuk writes clearly that the “gifts” are given to the Kohanim as “remuneration” for their Temple service, so that it stands to reason that these “gifts” will have been the possession of the Kohanim, had they remained in their position of performing the Avodah.
Best wishes.
