Kautilya's Arthashastra 4.2.23

4.2.23

yanniṣṛṣṭam upajīveyus tad eṣāṃ divasasaṃjātaṃ saṃkhyāya vaṇik sthāpayet //

It is the duty of the trader to calculate the daily earnings of middlemen and to fix that amount on which they are authorised to live; for whatever income falls between sellers and purchasers (i.e., brokerage) is different from profit.