Zheng-tong: Year 13, Month 2, Day 5
9 Mar 1448
Previously, as many of the horses presented in lieu of labour by the yi people under Yun-nan could not be used, the three offices of Yun-nan memorialized and obtained approval for a proposal that for each horse due, an amount of 13 liang of silver be levied, and that the silver so obtained be stored in the treasury until there was an expedition, when it could be provided to the army for purchasing horses and paying for training. At this time, the yi people all complained that the silver price was too heavy and that they wished to continue to pay in horses. The Emperor, considering that they were distant yi, allowed this for their convenience.
Zhong-yang Yan-jiu yuan Ming Shi-lu, volume 29, page 3158