《元史·本纪·卷十七》
最初的规定，每炼银子一两，免除役夫的田税五斗，如今民力日益困敝，每炼出一两拟免征一石。 
初制，炼银一两，免役夫田租五斗，今民力日困，每两拟免一石。 
According to the original regulations, every liang of silver refined should exempt five dou of land tax for a laborer. Due to the increasing depletion of human resources, the tax-exemption should be one shi for every liang refined.