Kautilya's Arthashastra 5.3.44

5.3.44

evaṃ rājapaṇyayogavikrayo vetanapratyādānaṃ ca bhavati //

When starting on a military tour, the king shall put his army in action. On such occasions, spies, under the garb of merchants, shall supply to military stations all kinds of merchandise for double the quantity of the same to be repaid in future. Thus not only is there afforded an opportunity for the sale of the king's merchandise, but also is there a way opened for a good return for the wages paid.