The wealth from slaves in Essos would be too much for the Iron Bank to boycott(think of some of our banks with old ties to slavery). How would they justify this to the Sealord or Braavosi ? If not Braavos, what city is truly “free” in known Essos?

I don’t think the Iron Bank would trade in slaves itself, or lend credit to others to do the same. (For one thing, given the way that the Sealord came crashing down on the Goodheart, such investments would be highly insecure in Braavos) 

However, lending to banks in Myr or Lys or Tyrosh or Volantis who might lend to people trading in slaves, or lending to ship captains who might later use their ship to transport slaves, they probably would do, on the grounds that they’re not directly involved in slavery.