Adam Clarke on Hebrews 7:2

Gave a tenth part of all - It was an ancient custom, among all the nations of the earth, to consecrate a part or tenth of the spoils taken in war to the objects of their worship. Many examples of this kind occur. This however was not according to any provision in law, but merely ad libitum, and as a eucharistic offering to those to whom they imagined they owed the victory. But neither Abraham's decimation, nor theirs, had any thing to do, either with tithes as prescribed under the Mosaic dispensation, or as claimed under the Christian.