If morality is real (or 'objective',or 'objectively real') then either 'slavery is good' is true or 'slavery is bad' is true.
Importantly if morality is real then 'slavery is bad' even if people - such as the ancient Romans - don't think so. (Note: In theory slavery could be objectively good, but I'm keeping things simple!)
That is to say our (or the Romans') moral judgement of slavery is independent of the [objective] morality of slavery . Hold that thought for later!
Now, as an example of something that definitely is an objective truth, let's take 2+2=4, (or more generally the mathematics which apllies in our universe which includes 2+2=4). Now suppose a genie changes the rule from 2+2=4 to 2+2=5. The effect would be catastrophic - very likely the universe would blow up as billions of extra things pop into existence each time a 2+2 addition happens.
The point is that if you change an objective fact it is noticeble - there is a real effect.
So let the genie change 'slavery is bad' to 'slavery is good' - what would be the effect of that? Would we immeditely restart the slave trade?
In fact, there'd be no effect at all. Our laws and our attitudes towards slavery derive from our collective judgement of slavery.
Remember what I i'd said above - "[our (or the Romans') moral judgement of slavery is independent of the [objective] morality of slavery.
The genie changed the objective morality of slavery (from bad to good) but he didn't change our collective judgement of slavery (slavery ids bad) upon which our laws attitudes are based so nothing actually changes.
In other words it doesn't make any difference if slavery is objectively bad (as per pre-genie change) or objectivly good (post-genie change).
But if something objective changes it does make difference, as discussed in the case of 2+2=5. So there is a seeming paradox, which can be resolved by realising that objective morality can't change because it does not exist.