Moses' Teachings say that members of the tribe of Levi who become priests must receive a tenth of everything from the people. The priests collect it from their own people, Abraham's descendants.
Although Melchizedek was not from the tribe of Levi, he received a tenth of everything from Abraham. Then Melchizedek blessed Abraham, who had God's promises.
No one can deny that the more important person blesses the less important person.
Priests receive a tenth of everything, but they die. Melchizedek received a tenth of everything, but we are told that he lives.
We could even say that when Abraham gave Melchizedek a tenth of everything, Levi was giving a tenth of everything. Levi gave, although later his descendants would receive a tenth of everything.
Even though Levi had not yet been born, he was in the body of Abraham when Melchizedek met him.
The people established the Levitical priesthood based on instructions they received. If the work of the Levitical priests had been perfect, we wouldn't need to speak about another kind of priest. However, we speak about another kind of priest, a priest like Melchizedek, not a Levitical priest like Aaron.
When a different kind of priesthood is established, the regulations for those priests are different.
The priest whom we are talking about was a member of a different tribe. No one from that tribe ever served as a priest at the altar.
Everyone knows that our Lord came from the tribe of Judah. Moses never said anything about priests coming from that tribe.
The regulations were different. This became clear when a different priest who is like Melchizedek appeared.