How does the Dhamma reconcile slavery? Since one of the aspects of wrong livelihood is human trafficking, why do we frequently read in the Canon instances where a slave should be good to his master, a master should be good to his slaves.
I think you are mistaking servant for slave.


  In five ways should a master minister to his servants and employees
  
  (i) by assigning them work according to their ability,
  
  (ii) by supplying them with food and with wages,
  
  (iii) by tending them in sickness,
  
  (iv) by sharing with them any delicacies,
  
  (v) by granting them leave.
  
  - Singalovada Sutta


This neither fits the description of a master-slave relationship nor does it involve any human trafficking. Therefore, it doesn't come under wrong livelihood.