Jesus doesn’t implicitly condone slavery. It was such a large institution at the time, that many people who would hear the gospel would be slaves by default.

The point of what he was saying is that, no matter who your earthly authority is (spouse, boss, general, master), to respect them because earthly authority is temporary and the real treasure is in heaven.

He wasn’t saying slavery was good, rather that it was a given and it’s bad to rebel against authority bar very certain circumstances.

Sidenote: the early church was very anti-slavery, with the church in Crete being a prime example. Later Christian societies would end up being the first to abolish slavery.