Jia-jing: Year 12, Month 2, Day 20
15 Mar 1533
It had initially been ordered that when the foreign yi came to offer local products in tribute, the border officials were to examine their goods and submit a register. The Ministry of Rites would subsequently receive the goods and issue rewards in accordance with the register. The envoys were permitted to freely trade those goods which were not on the register. When the offering of tribute was completed, if they still had goods, they were instructed to take them home. If the envoys were willing to have the goods go to the treasury, the Ministry memorialized and paper money was provided in exchange. At the end of the Zheng-de reign (1506-21), because of cunning yi and wily officials, there was no profit in trading but there were still the regulations that when envoys had trade goods remaining, officials would provide thin silks and paper money in exchange at the market value of the goods...
Zhong-yang Yan-jiu yuan Ming Shi-lu, volume 78, page 3398/99