One of the major conflicts in the 19th century was about slavery or the abolishing of slavery (officially...). But the simple fact to allow it or not allow it seems (IMO) not enough. Just think of Mauretania, which was the last country to abolish slavery by the end of the 20th century (again "officially"). Basically I would distinguish 4 levels of slavery:
1. slavery forbidden
That would mean, no slavery allowed at all. There should be a negative modifier concerning diplomatic interactions with states with a higher slavery level. Slaves in newly conquered/colonized provinces should be turned to workers/farmers immediately
2. slavery tolerated, trade banned, no new slaves
More or less self explaining, the number of slaves in a province should slowly decline (moved to workers/farmers)
3. slave trade tolerated
For a country with this setting, slaves should appear as trade goods on the world market. Low level populations with bad economic situation can become slaves.
4. active slaver
Additional to level 3, a percentage (10%?) of the population of newly conquered provinces can be moved to the "slave pool" Also there would be the possibility for military units to raid uncontroled provinces for slaves...
Additionally slavery can be considered a war goal: For exampel Oman attacking Yemen to capture 10000 slaves?, or maybe England attacking Oman to force them to abolish slavery?
The level of slavery could be changed either as a social reform or in the case of a peace negociation. According to actual western attitude towards slavery I would not recomend an increase of slavery level....